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G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER

G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor

G11B5/74—Record carriers characterised by the form, e.g. sheet shaped to wrap around a drum

G—PHYSICS

G11—INFORMATION STORAGE

G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER

G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor

G11B5/62—Record carriers characterised by the selection of the material

G11B5/64—Record carriers characterised by the selection of the material comprising only the magnetic material without bonding agent

G11B5/66—Record carriers characterised by the selection of the material comprising only the magnetic material without bonding agent consisting of several layers

G—PHYSICS

G11—INFORMATION STORAGE

G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER

G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor

G11B2005/0002—Special dispositions or recording techniques

G—PHYSICS

G11—INFORMATION STORAGE

G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER

G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor

G11B5/74—Record carriers characterised by the form, e.g. sheet shaped to wrap around a drum

G11B5/82—Disk carriers

Abstract

A magnetic recording medium is provided with at
least one exchange layer structure, and a magnetic
layer (9) formed on the exchange layer structure. The
exchange layer structure includes a ferromagnetic layer
(7) and a non-magnetic coupling layer (8) provided on
the ferromagnetic layer (7) and under the magnetic
layer (9).

Description

BACKGROUND OF THE INVENTION1. Field of the Invention

The present invention generally relates to
magnetic recording media and magnetic storage
apparatuses, and more particularly to a magnetic
recording medium and a magnetic storage apparatus
which are suited for high-density recording. The
present invention also relates to a recording method
for magnetically recording information on a magnetic
recording medium, and to a method of producing such
a magnetic recording medium.

2. Description of the Related Art

The recording density of longitudinal
magnetic recording media, such as magnetic disks,
has been increased considerably, due to the
reduction of medium noise and the development of
magnetoresistive and high-sensitivity spin-valve
heads. A typical magnetic recording medium is
comprised of a substrate, an underlayer, a magnetic
layer, and a protection layer which are successively
stacked in this order. The underlayer is made of Cr
or a Cr-based alloy, and the magnetic layer is made
of a Co-based alloy.

Various methods have been proposed to
reduce the medium noise. For example, Okamoto et
al., "Rigid Disk Medium For 5 Gbit/in2 Recording",
AB-3, Intermag '96 Digest proposes decreasing the
grain size and size distribution of the magnetic
layer by reducing the magnetic layer thickness by
the proper use of an underlayer made of CrMo, and a
U.S. Patent No.5,693,426 proposes the use of an
underlayer made of NiAl. Further, Hosoe et al.,
"Experimental Study of Thermal Decay in High-Density
Magnetic Recording Media", IEEE Trans. Magn. Vol.33,
1528 (1997), for example, proposes the use of an
underlayer made of CrTiB. The underlayers described
above also promote c-axis orientation of the
magnetic layer in a plane which increases the
remanent magnetization and the thermal stability of
written bits. In addition, proposals have been made
to reduce the thickness of the magnetic layer, to
increase the resolution or to decrease the
transition width between written bits. Furthermore,
proposals have been made to decrease the exchange
coupling between grains by promoting more Cr
segregation in the magnetic layer which is made of
the CoCr-based alloy.

However, as the grains of the magnetic
layer become smaller and more magnetically isolated
from each other, the written bits become unstable
due to thermal activation and to demagnetizing
fields which increase with linear density. Lu et
al., "Thermal Instability at 10 Gbit/in2 Magnetic
Recording", IEEE Trans. Magn. Vol.30, 4230 (1994)
demonstrated, by micromagnetic simulation, that
exchange-decoupled grains having a diameter of 10 nm
and ratio KuV/kBT-60 in 400 kfci di-bits are
susceptible to significant thermal decay, where Ku
denotes the magnetic anisotropy constant, V denotes
the average magnetic grain volume, kB denotes the
Boltzmann constant, and T denotes the temperature.
The ratio KuV/kBT is also referred to as a thermal
stability factor.

It has been reported in Abarra et al.,
"Thermal Stability of Narrow Track Bits in a 5
Gbit/in2 Medium", IEEE Trans. Magn. Vol.33, 2995
(1997) that the presence of intergranular exchange
interaction stabilizes written bits, by MFM studies
of annealed 200 kfci bits on a 5 Gbit/in2
CoCrPtTa/CrMo medium. However, more grain
decoupling is essential for recording densities of
20 Gbit/in2 or greater.

The obvious solution has been to increase
the magnetic anisotropy of the magnetic layer. But
unfortunately, the increased magnetic anisotropy
places a great demand on the head write field which
degrades the "overwrite" performance which is the
ability to write over previously written data.

In addition, the coercivity of thermally
unstable magnetic recording medium increases rapidly
with decreasing switching time, as reported in He et
al., "High Speed Switching in Magnetic Recording
Media", J. Magn. Magn. Mater. Vol.155, 6 (1996), for
magnetic tape media, and in J. H. Richter, "Dynamic
Coervicity Effects in Thin Film Media", IEEE Trans.
Magn. Vol.34, 1540 (1997), for magnetic disk media.
Consequently, the adverse effects are introduced in
the data rate, that is, how fast data can be written
on the magnetic layer and the amount of head field
required to reverse the magnetic grains.

On the other hand, another proposed method
of improving the thermal stability increases the
orientation ratio of the magnetic layer, by
appropriately texturing the substrate under the
magnetic layer. For example, Akimoto et al.,
"Relationship Between Magnetic Circumferential
Orientation and Magnetic Thermal Stability", J. Magn.
Magn. Mater. vol.193, pp.240-242(1999), in press,
report through micromagnetic simulation, that the
effective ratio KuV/kBT is enhanced by a slight
increase in the orientation ratio. This further
results in a weaker time dependence for the
coercivity which improves the overwrite performance
of the magnetic recording medium, as reported in
Abarra et al., "The Effect of Orientation Ratio on
the Dynamic Coercivity of Media for >15 Gbit/in2
Recording", IEEE Trans. Magn. vol.35, pp.2709-2711,
1999.

Furthermore, keepered magnetic recording
media have been proposed for thermal stability
improvement. The keeper layer is made up of a
magnetically soft layer parallel to the magnetic
layer. This soft layer can be disposed above or
below the magnetic layer. Oftentimes, a Cr
isolation layer is interposed between the soft layer
and the magnetic layer. The soft layer reduces the
demagnetizing fields in written bits on the magnetic
layer. However, coupling the magnetic layer to a
continuously-exchanged coupled soft layer defeats
the purpose of decoupling the grains of the magnetic
layer. As a result, the medium noise increases.

Various methods have been proposed to
improve the thermal stability and to reduce the
medium noise. However, there was a problem in that
the proposed methods do not provide a considerable
improvement of the thermal stability of written bits,
thereby making it difficult to greatly reduce the
medium noise. In addition, there was another
problem in that some of the proposed methods
introduce adverse effects on the performance of the
magnetic recording medium due to the measures taken
to reduce the medium noise.

More particularly, in order to obtain a
thermally stable performance of the magnetic
recording medium, it is conceivable to (i) increase
the magnetic anisotropy constant Ku, (ii) decrease
the temperature T or, (iii) increase the grain
volume V of the magnetic layer. However, measure
(i) increases the coercivity, thereby making it more
difficult to write information on the magnetic layer.
In addition, measure (ii) is impractical since in
magnetic disk drives, for example, the operating
temperature may become greater than 60° C.
Furthermore, measure (iii) increases the medium
noise as described above. As an alternative for
measure (iii), it is conceivable to increase the
thickness of the magnetic layer, but this would lead
to deterioration of the resolution.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the
present invention to provide a novel and useful
magnetic recording medium, magnetic storage
apparatus, recording method and method of producing
magnetic recording medium, in which the problems
described above are eliminated.

Another and more specific object of the
present invention is to provide a magnetic recording
medium, a magnetic storage apparatus, a recording
method and a method of producing a magnetic
recording medium, which can improve the thermal
stability of written bits without increasing the
medium noise, so as to enable reliable high-density
recording without introducing adverse effects on the
performance of the magnetic recording medium, that
is, unnecessarily increasing the magnetic anisotropy.

Still another object of the present
invention is to provide a magnetic recording medium
comprising at least one exchange layer structure,
and a magnetic layer formed on said exchange layer
structure, where said exchange layer structure
comprises a ferromagnetic layer, and a non-magnetic
coupling layer provided on said ferromagnetic layer
and under said magnetic layer. According to the
magnetic recording medium of the present invention,
it is possible to provide a magnetic recording
medium which can improve the thermal stability of
written bits, so as to enable reliable high-density
recording without degrading the overwrite
performance.

A further object of the present invention
is to provide a magnetic recording medium comprising
a substrate, an underlayer disposed above said
substrate, and a magnetic layer structure including
at least a bottom ferromagnetic layer provided on
the underlayer and having a remanent magnetization
and thickness product Mriδi, and a top ferromagnetic
layer disposed above the bottom ferromagnetic layer
and having a remanent magnetization and thickness
product Mrjδj, wherein a relationship Mrδ≒Σ(Mriδi-
Mrjδj) is satisfied, where Mrδ denotes a total
remanent magnetization and thickness product of the
magnetic layer structure, so that magnetization
directions of adjacent ferromagnetic layers in the
magnetic layer structure are closely antiparallel.

Another object of the present invention is
to provide a method of magnetically recording
information on a magnetic recording medium,
comprising a step of switching magnetization
direction of at least one of ferromagnetic layers
which form a magnetic layer structure of the
magnetic recording medium and have antiparallel
magnetization directions.

Still another object of the present
invention is to provide a method of producing a
magnetic recording medium having a substrate, an
underlayer and a magnetic layer structure,
comprising the steps of (a) forming the magnetic
layer structure to include at least a bottom
ferromagnetic layer provided on the underlayer and
having a remanent magnetization and thickness
product Mriδi, and a top ferromagnetic layer
disposed above the bottom ferromagnetic layer and
having a remanent magnetization and thickness
product Mrjδj, wherein a relationship Mrδ≒Σ(Mriδi-
Mrjδj) is satisfied, where Mrδ denotes a total
remanent magnetization and thickness product of the
magnetic layer structure, so that magnetization
directions of adjacent ferromagnetic layers in the
magnetic layer structure are closely antiparallel,
and (b) forming the underlayer and the magnetic
structure by sequential (continuous) sputtering.

A further object of the present invention
is to provide a magnetic recording medium comprising
at least one exchange layer structure and a magnetic
layer provided on the exchange layer structure, the
exchange layer structure including a ferromagnetic
layer and a non-magnetic coupling layer provided on
the ferromagnetic layer, and a magnetic bonding
layer provided between the ferromagnetic layer and
the non-magnetic coupling layer and/or between the
non-magnetic coupling layer and the magnetic layer,
the magnetic bonding layer having a magnetization
direction parallel to the ferromagnetic layer and
the magnetic layer. According to the magnetic
recording medium of the present invention, it is
possible to provide a magnetic recording medium
which can improve the thermal stability of written
bits, so as to enable reliable high-density
recording without degrading the overwrite
performance.

Another object of the present invention is
to provide a magnetic recording medium characterized
by at least one exchange layer structure; and a
magnetic layer formed on the exchange layer
structure, the exchange layer structure comprising a
ferromagnetic layer, and a non-magnetic coupling
layer provided on the ferromagnetic layer and under
the magnetic layer, the ferromagnetic layer and the
magnetic layer having antiparallel magnetizations,
and the non-magnetic coupling layer being made of a
Ru-M3 alloy, where M3 is an added element or alloy,
and a lattice mismatch between the non-magnetic
coupling layer and the magnetic layer and the
ferromagnetic layer respectively disposed above and
below the non-magnetic coupling layer is adjusted to
approximately 6% or less by addition of M3.
According to the magnetic recording medium of the
present invention, it is possible to provide a
magnetic recording medium which can improve the
thermal stability of written bits, so as to enable
reliable high-density recording without degrading
the overwrite performance, and to improve the
recording resolution by improving the in-plane
crystal orientation of the magnetic layer.

Still another object of the present
invention is to provide a magnetic recording medium
characterized by at least one exchange layer
structure; and a magnetic layer formed on the
exchange layer structure, the exchange layer
structure comprising a ferromagnetic layer, and a
non-magnetic coupling layer provided on the
ferromagnetic layer and under the magnetic layer,
the ferromagnetic layer and the magnetic layer
having antiparallel magnetizations, the non-magnetic
coupling layer being made of a Ru-M3 alloy, where M3
= Co, Cr, Fe, Ni, Mn or alloys thereof. According
to the magnetic recording medium of the present
invention, it is possible to provide a magnetic
recording medium which can improve the thermal
stability of written bits, so as to enable reliable
high-density recording without degrading the
overwrite performance, and to improve the recording
resolution by improving the in-plane crystal
orientation of the magnetic layer.

A further object of the present invention
is to provide a magnetic recording medium comprising
a substrate, an underlayer disposed above the
substrate, and a magnetic recording layer disposed
above the underlayer, where the magnetic recording
layer has a multi-layer structure which is separated
into at least upper and lower layers by a non-magnetic
separation layer, the non-magnetic
separation layer is made of a material selected from
a group of Ru, Rh, Ir and alloys thereof, and the
upper and lower layers of the multi-layer structure
separated by the non-magnetic separation layer have
magnetization directions which are mutually parallel.
According to the magnetic recording medium of the
present invention, the non-magnetic separation layer
which is made of Ru or the like and has a
predetermined thickness maintains the magnetic
coupling of magnetic recording layers above and
below the non-magnetic separation layer to a
mutually parallel state. Hence, it is possible to
realize a magnetic recording medium having low noise
and desired thermal stability. Compared to the
conventional magnetic recording medium, this
magnetic recording medium has a high reliability and
is suited for high-density recording.

Another object of the present invention is
to provide a magnetic recording medium comprising at
least one exchange layer structure and a magnetic
layer provided on the exchange layer structure,
where the exchange layer structure includes a
ferromagnetic layer and a non-magnetic coupling
layer provided on the ferromagnetic layer, at least
one of the ferromagnetic layer and the magnetic
layer has a granular layer structure in which
ferromagnetic crystal grains are uniformly
distributed within a non-magnetic base material.
According to the magnetic recording medium of the
present invention, it is possible to provide a
magnetic recording medium which can improve the
thermal stability of written bits, so as to enable
reliable high-density recording without degrading
the overwrite performance. By employing the
granular layer structure which is effective in
reducing noise for at least the ferromagnetic layer
of the exchange layer structure and the magnetic
layer which is provided on the exchange layer
structure, it is possible to further reduce the
medium noise while further improving the thermal
stability of the written bits.

The magnetic recording medium may comprise
at least a first exchange layer structure and a
second exchange layer structure provided between the
first exchange layer structure and the magnetic
layer, where the first and second exchange layer
structures have a granular layer structure, the
second exchange layer structure has a granular layer
with a magnetic anisotropy smaller than that of a
granular layer of the first exchange layer structure,
and the granular layers of the first and second
exchange layer structures have magnetization
directions which are mutually antiparallel.

The magnetic recording medium may comprise
at least a first exchange layer structure and a
second exchange layer structure provided between the
first exchange layer structure and the magnetic
layer, where the first and second exchange layer
structures have a granular layer structure, the
second exchange layer structure has a granular layer
with a remanence magnetization and thickness product
smaller than that of a granular layer of the first
exchange layer structure, and the granular layers of
the first and second exchange layer structures have
magnetization directions which are mutually
antiparallel.

Still another object of the present
invention is to provide a magnetic storage apparatus
comprising at least one magnetic recording medium of
any one of the types described above. According to
the magnetic storage apparatus of the present
invention, it is possible to provide a magnetic
storage apparatus which can improve the thermal
stability of written bits, so as to enable a
reliable high-density recording without introducing
adverse effects on the performance of the magnetic
recording medium.

Other objects and further features of the
present invention will be apparent from the
following detailed description when read in
conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing
an important part of a first embodiment of the
magnetic recording medium according to the present
invention;

FIG. 2 is a cross sectional view showing
an important part of a second embodiment of the
magnetic recording medium according to the present
invention;

FIG. 3 is a diagram showing an in-plane
magnetization curve of a single CoPt layer having a
thickness of 10 nm on a Si substrate;

FIG. 4 is a diagram showing an in-plane
magnetization curve of two CoPt layers separated by
a Ru layer having a thickness of 0.8 nm;

FIG. 5 is a diagram showing an in-plane
magnetization curve of two CoPt layers separated by
a Ru layer having a thickness of 1.4 nm;

FIG. 6 is a diagram showing an in-plane
magnetization curve two CoCrPt layers separated by a
Ru having a thickness of 0.8 nm;

FIG. 7 is a cross sectional view showing
an important part of an embodiment of the magnetic
storage apparatus according to the present
invention;

FIG. 8 is a plan view showing the
important part of the embodiment of the magnetic
storage apparatus;

FIG. 9 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having a single CoCrPtB layer grown on a NiAl layer
on glass;

FIG. 10 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having two ferromagnetic layers of CoCrPtB separated
by a Ru layer having a thickness of 0.8 nm on a NiP
coated Al-Mg substrate;

FIG. 11 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having two ferromagnetic layers of CoCrPtB separated
by a Ru layer on a NiP coated Al substrate;

FIG. 12 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having three ferromagnetic layers of CoCrPtB
separated by a Ru layer between each two adjacent
CoCrPtB layers on a NiP coated Al substrate;

FIG. 13 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having two negatively coupled ferromagnetic layers
of CoCrPtB separated by a Ru layer, on a NiAl coated
glass substrate;

FIG. 14 is a diagram showing an in-plane
magnetization curve shown in FIG. 13 in comparison
with a magnetic recording medium having a single
ferromagnetic layer of CoCrPtB on a NiAl coated
glass substrate;

FIG. 15 is a diagram showing signal decays
of the magnetic recording media having two and three
ferromagnetic layers, in comparison with a signal
decay of the magnetic recording medium having the
single ferromagnetic layer;

FIG. 16 is a diagram showing M-H curves of
the magnetic recording medium having the two
negatively coupled ferromagnetic layers at different
temperatures;

FIG. 17 is a diagram showing the
temperature dependence of the coercivity for the
magnetic recording medium having the characteristics
shown in FIG. 16;

FIG. 18 is a diagram showing the PW50
dependence on the effective and total ferromagnetic
layer thickness of the magnetic recording media
having one, two and three ferromagnetic layers;

FIG. 19 is a diagram showing the effective
thickness dependence of the change in isolated wave
medium SNR;

FIG. 20 is a diagram showing the general
construction of a magnetic recording medium
producing apparatus;

FIG. 21 is a diagram showing the
dependence of isolated wave output on magnetic layer
thickness;

FIG. 22 is a diagram showing the
temperature dependence of high-frequency SNR;

FIG. 23 is a diagram showing a relation
ship of the isolated wave medium SNR Siso/Nm and the
sputtering rate of Ru;

FIG. 24 is a cross sectional view showing
an important part of a fourth embodiment of the
magnetic recording medium according to the present
invention;

FIG. 25 is a diagram for explaining in-plane
characteristics of two CoCr-based alloy layers
separated by Ru;

FIG. 26 is a cross sectional view showing
an important part of a fifth embodiment of the
magnetic recording medium according to the present
invention;

FIG. 27 is a diagram showing a
magnetization curve which is obtained when pure Ru
is used for a non-magnetic coupling layer of the
magnetic recording medium;

FIG. 28 is a diagram showing a
magnetization curve which is measured by a vertical
Kerr looper while applying a magnetic field in a
perpendicular direction with respect to a sample
surface;

FIG. 29 is a cross sectional view showing
an important part of a sixth embodiment of the
magnetic recording medium according to the present
invention;

FIG. 30 is a diagram showing the
relationship of a ratio Siso/Nm of the isolated wave
output (Siso) and medium noise (Nm) of the sixth
embodiment of the magnetic recording medium and the
thickness of a Ru non-magnetic separation layer;

FIG. 31 is a diagram showing the
relationship of a thickness ratio of first and
second magnetic recording layers and the ratio
Siso/Nm of the isolated wave output (Siso) and
medium noise (Nm);

FIG. 32 is a diagram showing the
relationship of the thickness ratio of the first and
second magnetic recording layers and a ratio S/Nt of
an output (S) and noise (Nt); and

FIG. 33 is a cross sectional view showing
an important part of a seventh embodiment of the
magnetic recording medium according to the present
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will hereinafter be given of
embodiments of the present invention, by referring
to the drawings.

First, a description will be given of the
operating principle of the present invention.

The present invention submits the use of
layers with antiparallel magnetization structures.
For example, S. S. P. Parkin, "Systematic Variation
of the Strength and Oscillation Period of Indirect
Magnetic Exchange Coupling through the 3d, 4d, and
5d Transition Metals", Phys. Rev. Lett. Vol.67, 3598
(1991) describes several magnetic transition metals
such as Co, Fe and Ni that are coupled through thin
non-magnetic interlayers such as Ru and Rh. On the
other hand, a U.S. Patent No.5,701,223 proposes a
spin-valve which employs the above described layers
as laminated pinning layers to stabilize the sensor.

For a particular Ru or Ir layer thickness
between two ferromagnetic layers, the magnetizations
can be made parallel or antiparallel. For example,
for a structure made up of two ferromagnetic layers
of different thickness with antiparallel
magnetizations, the effective grain size of a
magnetic recording medium can be increased without
significantly affecting the resolution. A signal
amplitude reproduced from such a magnetic recording
medium is reduced due to the opposite magnetizations,
but this can be rectified by adding another layer of
appropriate thickness and magnetization direction,
under the laminated magnetic layer structure, to
thereby cancel the effect of one of the layers. As
a result, it is possible to increase the signal
amplitude reproduced from the magnetic recording
medium, and to also increase the effective grain
volume. Thermally stable written bits can therefore
be realized.

The present invention increases the
thermal stability of written bits by exchange
coupling the magnetic layer to another ferromagnetic
layer with an opposite magnetization or, by a
laminated ferrimagnetic structure. The
ferromagnetic layer or the laminated ferrimagnetic
structure is made up of exchange-decoupled grains as
the magnetic layer. In other words, the present
invention uses an exchange pinning ferromagnetic
layer or a ferrimagnetic multilayer to improve the
thermal stability performance of the magnetic
recording medium.

FIG. 1 is a cross sectional view showing
an important part of a first embodiment of a
magnetic recording medium according to the present
invention.

The magnetic recording medium includes a
non-magnetic substrate 1, a first seed layer 2, a
NiP layer 3, a second seed layer 4, an underlayer 5,
a non-magnetic intermediate layer 6, a ferromagnetic
layer 7, a non-magnetic coupling layer 8, a magnetic
layer 9, a protection layer 10, and a lubricant
layer 11 which are stacked in the order shown in FIG.
1.

For example, the non-magnetic substrate 1
is made of Al, Al alloy or glass. This non-magnetic
substrate 1 may or may not be mechanically textured.
The first seed layer 2 is made of Cr or Ti, for
example, especially in the case where the non-magnetic
substrate 1 is made of glass. The NiP
layer 3 is preferably oxidized and may or may not be
mechanically textured. The second seed layer 4 is
provided to promote a (001) or a (112) texture of
the underlayer 5 when using a B2 structure alloy
such as NiAl and FeAl for the underlayer 5. The
second seed layer 4 is made of an appropriate
material similar to that of the first seed layer 2.

In a case where the magnetic recording
medium is a magnetic disk, the mechanical texturing
provided on the non-magnetic substrate 1 or the NiP
layer 3 is made in a circumferential direction of
the disk, that is, in a direction in which tracks of
the disk extend.

The non-magnetic intermediate layer 6 is
provided to further promote epitaxy, narrow the
grain distribution of the magnetic layer 9, and
orient the anisotropy axes of the magnetic layer 9
along a plane parallel to the recording surface of
the magnetic recording medium. This non-magnetic
intermediate layer 6 is made of a hcp structure
alloy such as CoCr-M, where M = B, Mo, Nb, Ta, W, Cu
or alloys thereof, and has a thickness in a range of
1 to 5 nm.

The ferromagnetic layer 7 is made of Co,
Ni, Fe, Co-based alloy, Ni-based alloy, Fe-based
alloy or the like. In other words, alloys such as
CoCrTa, CoCrPt and CoCrPt-M, where M = B, Mo, Nb, Ta,
W, Cu or alloys thereof may be used for the
ferromagnetic layer 7. This ferromagnetic layer 7
has a thickness in a range of 2 to 10 nm. The non-coupling
magnetic layer 8 is made of Ru, Ir, Rh, Cr,
Cu, Ru-based alloy, Ir-based alloy, Rh-based alloy,
Cu-based alloy, Cr-based alloy or the like. This
non-magnetic coupling layer 8 preferably has a
thickness in a range of 0.4 to 1.0 nm for
antiparallel coupling using Ru, and preferably on
the order of approximately 0.6 to 0.8 nm for an
antiparallel coupling using Ru. For this particular
thickness range of the non-magnetic coupling layer 8,
the magnetizations of the ferromagnetic layer 7 and
the magnetic layer 9 are antiparallel. The
ferromagnetic layer 7 and the non-magnetic coupling
layer 8 form an exchange layer structure.

For a ferromagnetic layer 7 made of a Fe-based
alloy, Cr forms a better non-magnetic coupling
layer 8. In this case, the Cr non-magnetic coupling
layer 8 has an optimum thickness of approximately
1.8 nm.

The magnetic layer 9 is made of Co or a
Co-based alloys such as CoCrTa, CoCrPt and CoCrPt-M,
where M = B, Mo, Nb, Ta, W, Cu or alloys thereof.
The magnetic layer 9 has a thickness in a range of 5
to 30 nm. Of course, the magnetic layer 9 is not
limited to a single-layer structure, and a multi-layer
structure may be used for the magnetic layer 9.

The protection layer 10 is made of C, for
example. In addition, the lubricant layer 11 is
made of an organic lubricant, for example, for use
with a magnetic transducer such as a spin-valve head.
The protection layer 10 and the lubricant layer 11
form a protection layer structure on the recording
surface of the magnetic recording medium.

Obviously, the layer structure under the
exchange layer structure is not limited to that
shown in FIG. 1. For example, the underlayer 5 may
be made of Cr or Cr-based alloy and formed to a
thickness in a range of 5 to 40 nm on the substrate
1, and the exchange layer structure may be provided
on this underlayer 5.

Next, a description will be given of a
second embodiment of the magnetic recording medium
according to the present invention.

FIG. 2 is a cross sectional view showing
an important part of the second embodiment of the
magnetic recording medium. In FIG. 2, those parts
which are the same as those corresponding parts in
FIG. 1 are designated by the same reference numerals,
and a description thereof will be omitted.

In this second embodiment of the magnetic
recording medium, the exchange layer structure
includes two non-magnetic coupling layers 8 and 8-1,
and two ferromagnetic layers 7 and 7-1, which form a
ferrimagnetic multilayer. This arrangement
increases the effective magnetization and signal,
since the magnetizations of the two non-magnetic
coupling layers 8 and 8-1 cancel each other instead
of a portion of the magnetic layer 9. As a result,
the grain volume and thermal stability of
magnetization of the magnetic layer 9 are
effectively increased. More bilayer structures made
up of the pair of ferromagnetic layer and non-magnetic
coupling layer may be provided additionally
to increase the effective grain volume, as long as
the easy axis of magnetization are appropriately
oriented for the subsequently provided layers.

The ferromagnetic layer 7-1 is made of a
material similar to that of ferromagnetic layer 7,
and has a thickness range selected similarly to the
ferromagnetic layer 7. In addition, the non-magnetic
coupling layer 8-1 is made of a material
similar to that of the non-magnetic coupling layer 8,
and has a thickness range selected similarly to the
non-magnetic coupling layer 8. Within the
ferromagnetic layers 7-1 and 7, the c-axes are
preferably in-plane and the grain growth columnar.

In this embodiment, the magnetic
anisotropy of the ferromagnetic layer 7-1 is
preferably higher than that of the ferromagnetic
layer 7. However, the magnetic anisotropy of the
ferromagnetic layer 7-1 may be the same as or, be
higher than that of, the magnetic layer 9.

Furthermore, a remanent magnetization and
thickness product of the ferromagnetic layer 7 may
be smaller than that of the ferromagnetic layer 7-1.

FIG. 3 is a diagram showing an in-plane
magnetization curve of a single CoPt layer having a
thickness of 10 nm on a Si substrate. In FIG. 3,
the ordinate indicates the magnetization (emu), and
the abscissa indicates the magnetic field (Oe).
Conventional magnetic recording media show a
behavior similar to that shown in FIG. 3.

FIG. 4 is a diagram showing an in-plane
magnetization curve of two CoPt layers separated by
a Ru layer having a thickness of 0.8 nm, as in the
case of the first embodiment of the magnetic
recording medium. In FIG. 4, the ordinate indicates
the magnetization (Gauss), and the abscissa
indicates the magnetic field (Oe). As may be seen
from FIG. 4, the loop shows shifts near the magnetic
field which indicate the antiparallel coupling.

FIG. 5 is a diagram showing an in-plane
magnetization curve of two CoPt layers separated by
a Ru layer having a thickness of 1.4 nm. In FIG. 5,
the ordinate indicates the magnetization (emu), and
the abscissa indicates the magnetic field (Oe). As
may be seen from FIG. 5, the magnetizations of the
two CoPt layers are parallel.

FIG. 6 is a diagram showing an in-plane
magnetization curve for two CoCrPt layers separated
by a Ru having a thickness of 0.8 nm, as in the case
of the second embodiment of the magnetic recording
medium. In FIG. 6, the ordinate indicates the
magnetization (emu/cc), and the abscissa indicates
the field (Oe). As may be seen from FIG. 6, the
loop shows shifts near the field which indicate the
antiparallel coupling.

From FIGS. 3 and 4, it may be seen that
the antiparallel coupling can be obtained by the
provision of the exchange layer structure. In
addition, it may be seen by comparing FIG. 5 with
FIGS. 4 and 6, the non-magnetic coupling layer 8 is
desirably in the range of 0.4 to 1.0 nm in order to
achieve the antiparallel coupling.

Therefore, according to the first and
second embodiments of the magnetic recording medium,
it is possible to effectively increase the apparent
grain volume of the magnetic layer by the exchange
coupling provided between the magnetic layer and the
ferromagnetic layer via the non-magnetic coupling
layer, without sacrificing the resolution. In other
words, the apparent thickness of the magnetic layer
is increased with regard to the grain volume of the
magnetic layer so that a thermally stable medium can
be obtained, and in addition, the effective
thickness of the magnetic layer is maintained since
cancellation of signals especially from the bottom
layers is achieved. This allows higher linear
density recording that is otherwise not possible for
thick media. As a result, it is possible to obtain
a magnetic recording medium with reduced medium
noise and thermally stable performance.

Next, a description will be given of an
embodiment of a magnetic storage apparatus according
to the present invention, by referring to FIGS. 7
and 8. FIG. 7 is a cross sectional view showing an
important part of this embodiment of the magnetic
storage apparatus, and FIG. 8 is a plan view showing
the important part of this embodiment of the
magnetic storage apparatus.

As shown in FIGS. 7 and 8, the magnetic
storage apparatus generally includes a housing 13.
A motor 14, a hub 15, a plurality of magnetic
recording media 16, a plurality of recording and
reproducing heads 17, a plurality of suspensions 18,
a plurality of arms 19, and an actuator unit 20 are
provided within the housing 13. The magnetic
recording media 16 are mounted on the hub 15 which
is rotated by the motor 14. The recording and
reproducing head 17 is made up of a reproducing head
such as a MR or GMR head, and a recording head such
as an inductive head. Each recording and
reproducing head 17 is mounted on the tip end of a
corresponding arm 19 via the suspension 18. The
arms 19 are moved by the actuator unit 20. The
basic construction of this magnetic storage
apparatus is known, and a detailed description
thereof will be omitted in this specification.

This embodiment of the magnetic storage
apparatus is characterized by the magnetic recording
media 16. Each magnetic recording medium 16 has the
structure of the first or second embodiment of the
magnetic recording medium described above in
conjunction with FIGS. 1 and 2. Of course, the
number of magnetic recording media 16 is not limited
to three, and only one, two or four or more magnetic
recording media 16 may be provided. Further, each
magnetic recording medium 16 may have the structure
of any of the embodiments of the magnetic recording
medium which will be described later.

The basic construction of the magnetic
storage unit is not limited to that shown in FIGS. 7
and 8. In addition, the magnetic recording medium
used in the present invention is not limited to a
magnetic disk.

Next, a description will be given of
further features of the present invention, in
comparison with the conventional magnetic recording
medium having no exchange layer structure. In the
following description, the ferromagnetic layer of
the exchange layer structure and the magnetic layer
will also be referred to as ferromagnetic layers
forming a magnetic layer structure.

FIG. 9 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having a single layer of CoCrPtB grown on a NiAl
layer on glass. In FIG. 9, the ordinate indicates
the magnetization M (emu/cc), and the abscissa
indicates the magnetic field H (Oe). Similar M-H
curves are observed for a single Co-based layer
grown on a Cr underlayer on NiP coated Al substrate
or NiP coated glass substrate.

On the other hand, FIG. 10 is a diagram
showing an in-plane magnetization curve for a
magnetic recording medium having two ferromagnetic
layers of CoCrPtB separated by a Ru layer having a
thickness of 0.8 nm, sputtered on a NiP coated Al-Mg
substrate. In FIG. 10, the ordinate indicates the
magnetization M (emu/cc), and the abscissa indicates
the magnetic field H (Oe). As may be seen from FIG.
10, the magnetization M abruptly decreases when the
magnetic field H is around H=500 Oe which indicates
an exchange coupling field of approximately 1000 Oe.
The reduced magnetization M at H=0 evidences the
anti-parallel coupling.

The optimum Ru thickness for the negative
coupling can be determined not only by magnetometry
but also by spin stand methods. The reproduced
signal at low densities gives an indication of a
remanent magnetization and thickness product Mrδ,
where Mr denotes the remanent magnetization and δ
denotes the effective thickness of the CoCrPtB layer,
that is, the ferromagnetic layer of the magnetic
layer structure. If the Ru thickness is varied
while the thicknesses of the two CoCrPtB layers are
maintained constant, the reproduced signal shows a
dip at the optimum Ru thickness. The optimum Ru
thickness may depend on the magnetic materials and
the processing used to form the ferromagnetic layers
of the magnetic layer structure. For CoCrPt-based
alloys manufactured above 150°C, the antiparallel
coupling is induced for the Ru thickness in a range
of approximately 0.4 to 1.0 nm.

FIG. 11 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having two ferromagnetic layers of CoCrPtB separated
by a Ru layer, on a NiP coated Al substrate. In FIG.
11, the ordinate indicates the magnetization M
(emu/cc), and the abscissa indicates the magnetic
field H (Oe). FIG. 11 shows a case where a first
CoCrPtB layer closer to the substrate is 8 nm thick,
the Ru layer is 0.8 nm thick, and a second CoCrPtB
layer further away from the substrate is 20 nm thick.

In this case, antiparallel coupling is
observed but at higher negative magnetic fields.
Unless the demagnetizing fields inside bits are very
high, the antiparallel coupling is not completely
achieved and very high reproduced signals are
observed as the magnetizations in both the first and
second CoCrPtB layers point in essentially the same
direction. It is therefore necessary to reduce the
coercivity Hc of the first CoCrPtB layer by reducing
the thickness thereof or, by use of compositions
which result in a lower coercivity Hc. For CoCrPt-based
materials, the latter is usually achieved by
increasing the Cr content and/or reducing the Pt
content.

FIG. 12 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having three ferromagnetic layers of CoCrPtB
separated by a Ru layer between each two adjacent
CoCrPtB layers, on a NiP coated Al substrate. In
FIG. 12, the ordinate indicates the magnetization M
(emu/cc), and the abscissa indicates the magnetic
field H (Oe). FIG. 12 shows a case where first and
second CoCrPtB layers closer to the substrate are 6
nm thick, a top third CoCrPtB layer is 20 nm thick,
and the Ru layers between the first and second
CoCrPtB layers and between the second and third
CoCrPtB layers respectively are 0.8 nm thick. In
this case, the magnetization M drops when the
magnetic field H is H=500 Oe, which indicates that
one of the first through third CoCrPtB layers
reversed magnetization at positive fields. It is
likely the middle second CoCrPtB layer which
reversed magnetization since this middle second
CoCrPtB layer is subject to a stronger reversing
field due to the two interfaces. The interlayer
interaction is therefore 500 Oe greater than the
coercivity Hc of the middle second CoCrPtB layer.

However, at low negative magnetic fields,
the bottom first CoCrPtB layer starts reversing
magnetization, such that at approximately -1000 Oe,
the magnetization of only the top third CoCrPtB
layer is not reversed. Preferably, the bottom first
CoCrPtB layer should not reverse magnetization at
magnetic fields which are low compared to the
demagnetizing fields inside bits, and this may be
achieved for example by choosing the proper
thickness and/or composition for the bottom first
CoCrPtB layer. The magnetic recording medium which
has these three ferromagnetic layers tend to have
read-write performance which is better than the
magnetic recording medium which only has a single
ferromagnetic (magnetic) layer with no exchange
coupling. There is a possibility that the
reproduced signal will be reduced with time as more
grains change layer magnetization configuration from
parallel to antiparallel which is more stable.
However, a solitary wave media signal-to-noise ratio
(SNR) Siso/Nm of the magnetic recording medium is
expected to be maintained since the medium noise
level is also correspondingly reduced. Hence, the
bit error rate (BER) which is intimately related to
the isolated wave medium SNR Siso/Nm will not be
degraded.

FIG. 13 is a diagram showing an in-plane
magnetization curve for a magnetic recording medium
having two negatively coupled ferromagnetic layers
of CoCrPtB separated by a Ru layer, on a NiAl coated
glass substrate. In FIG. 13, the ordinate indicates
the magnetization M (emu/cc), and the abscissa
indicates the magnetic field H (Oe). As shown in
FIG. 13, the bottom CoCrPtB layer closer to the
substrate reverses magnetization even before the
magnetic field H becomes H=0 Oe.

FIG. 14 is a diagram showing an in-plane
magnetization curve shown in FIG. 13 in comparison
with a magnetic recording medium having a single
ferromagnetic layer of CoCrPtB on a NiAl coated
glass substrate fabricated similarly to the
recording medium having the two negatively coupled
ferromagnetic layers. In FIG. 14, the ordinate
indicates the magnetization M (emu/cc), and the
abscissa indicates the magnetic field H (Oe). In
FIG. 14, the in-plane magnetization curve shown in
FIG. 13 is indicated by a solid line, and an in-plane
magnetization curve for the recording medium
with the single ferromagnetic layer is indicated by
a dashed line. In FIG. 14, the saturation
magnetization is normalized so as to illustrate the
similarity of the M-H curve portions relevant to the
magnetic recording.

When a head saturates a portion of the
magnetic recording medium having the two negatively
coupled ferromagnetic layers, the magnetization of
both the two ferromagnetic layers is in the head
field direction, but as soon as the head field is no
longer applied, the bottom ferromagnetic layer
reverses magnetization and the situation inside a
bit would be similar to that of the magnetic
recording medium having the single ferromagnetic
layer. A read head only senses the resultant
magnetization. A person skilled in the art can
therefore tune the thickness, composition and
processing of the ferromagnetic layers, so that the
magnetic recording medium behaves similarly to the
conventional magnetic recording medium but with an
enhanced thermal stability.

FIG. 15 is a diagram showing signal decays
of the magnetic recording media having two and three
ferromagnetic layers, in comparison with a signal
decay of the magnetic recording medium having the
single ferromagnetic layer. In FIG. 15, the
ordinate indicates the signal decay (dB) of the
reproduced signal for 207 kfci bits, and the
abscissa indicates the time (s). In FIG. 15, ⋄
indicates the data of the magnetic recording medium
having the single CoCrPtB layer which is 10 nm thick,
 indicates the data of the magnetic recording
medium having the bottom first CoCrPtB layer which
is 10 nm thick, the Ru layer which is 0.8 nm thick
and the top second CoCrPtB layer which is 4 nm thick,
and □ indicates the data of the magnetic recording
medium having the bottom first CoCrPtB layer which
is 10 nm thick, the first Ru layer which is 0.8 nm
thick, the middle CoCrPtB layer which is 4 nm thick,
the second Ru layer which is 0.8 nm thick and the
top third CoCrPtB layer which is 4 nm thick. The
ferromagnetic layer compositions are all the same,
and the coercivity Hc measured with a Kerr
magnetometer are approximately 2700 Oe (214.8 kA/m)
and are similar. As may be seen from FIG. 15, the
magnetic recording media having two ferromagnetic
layers and three ferromagnetic layers show more
thermally stable characteristics as the effective
volume is increased, as compared to the magnetic
recording medium having the single ferromagnetic
layer and no exchange coupling.

FIG. 16 is a diagram showing M-H curves of
the magnetic recording medium having the two
negatively coupled ferromagnetic layers at different
temperatures. In FIG. 16, the ordinate indicates
the magnetization M (emu/cc), the abscissa indicates
the magnetic field H (Oe), and the data are shown
for three different temperatures which are 0°C, 25°C
and 75°C. A strong negative coupling is observed
over a wide temperature range, and covers the range
useful for magnetic recording media such as disks
and tapes.

FIG. 17 is a diagram showing the
temperature dependence of the coercivity for the
magnetic recording medium having the characteristics
shown in FIG. 16. In FIG. 17, the ordinate
indicates the coercivity Hc (Oe), and the abscissa
indicates the measured temperature (°C). In
addition, y=Hc and x=temperature in the expression
y=-15.47x+4019.7. The coercivity change with
temperature dHc/dT=15.5 Oe/°C and is less than that
of the magnetic recording medium having the single
ferromagnetic layer. A typical dHc/dT for the
magnetic recording medium having the single
ferromagnetic layer is 16 to 17 Oe/°C. Accordingly,
it may be clearly seen that the improved dHc/dT
value obtained for the magnetic recording medium
having the two negatively coupled ferromagnetic
layers primarily arises from the increased effective
volume.

FIG. 18 is a diagram showing the PW50
dependence on the effective and total ferromagnetic
layer thickness of the magnetic recording media
having two and three ferromagnetic layers, in
comparison with the PW50 dependence on the effective
and total ferromagnetic layer thickness of the
magnetic recording medium having the single
ferromagnetic layer. In FIG. 18, the ordinate
indicates the PW50 (ns), and the abscissa indicates
the effective and total ferromagnetic layer
thickness (nm). In FIG. 18, ◆ indicates the data
of the magnetic recording medium having the single
ferromagnetic layer, ▪ indicates the data of the
magnetic recording medium having two exchange-coupled
ferromagnetic layers, and ▵ indicates the
data of the magnetic recording medium having three
exchange-coupled ferromagnetic layers. The
thickness and composition of the ferromagnetic
layers are basically the same as those used to
obtain the data shown in FIG. 15. For the data on
the left side along the solid line, the thickness
used is the effective thickness, that is,
magnetization cancellation due to an antiparallel
configuration is assumed. Significant correlation
is observed validating the assumption. When the
total thickness of the ferromagnetic layer or layers
is used, the data shifts to the right along the
dotted line, which give unreasonably small PW50
values for the thicknesses involved when compared to
those of the magnetic recording medium having the
single ferromagnetic layer.

Therefore, although the writing resolution
may be degraded due to the increased media thickness,
the reading resolution is not, since cancellation of
the signals from the lower layers occurs which may
also explain the improved isolated wave medium SNR
Siso/Nm over the magnetic recording medium having
the single ferromagnetic layer. The isolated wave
medium SNR Siso/Nm of the magnetic recording medium
having the two exchange-coupled ferromagnetic layers
and very low effective Mrδ is especially improved
over that of the magnetic recording medium having
the single ferromagnetic layer. Such a very low
effective Mrδ can be achieved when the two
ferromagnetic layers have almost the same Mrδ. For
the magnetic recording medium having the three
exchange-coupled ferromagnetic layers, the
performance is enhanced when the sum of the
thicknesses of the bottom first and middle second
ferromagnetic layers is not so different from the
thickness of the top third ferromagnetic layer.
This phenomenon is consistent with a similar
phenomenon which occurs in double uncoupled layers
since the best thickness combination of the double
uncoupled layers is when both layers are of the same
thickness.

FIG. 19 is a diagram showing the effective
thickness dependence of the change in isolated wave
medium SNR. In FIG. 19, the ordinate indicates the
change ▵Siso/Nm (dB) of the isolated wave medium
SNR Siso/Nm, and the abscissa indicates the
effective thickness (nm) of the ferromagnetic layers.
In FIG. 19, the same symbols ◆, ▪ and ▵ are used
to indicate the data of the three different magnetic
recording media as in FIG. 18. It may be seen from
FIG. 19 that good isolated wave medium SNR Siso/Nm
is especially observed for the magnetic recording
medium having the two exchange-coupled ferromagnetic
layers with low Mrδ. Although the total thickness
of the ferromagnetic layers in this case becomes
greater than that of the magnetic recording medium
having the single ferromagnetic layer, the read-write
performance is hardly degraded, and in some
cases even improved.

The present inventors have also found that,
when at least one of the ferromagnetic layers of the
magnetic layer structure is made up of a plurality
of ferromagnetic layers which are in contact with
each other and ferromagnetically coupled, a good
performance is obtained especially when the lower
ferromagnetic layers is Cr-rich such that the Cr
content is 23 at% or greater, and the Cr content of
the upper ferromagnetic layer is less. This
indicates the crucial role of the lower
ferromagnetic layer. According to the experiments
conducted by the present inventors, it was found
that the noise arising from imperfections in the
lower ferromagnetic layer is effectively reduced due
to cancellation from the succeeding ferromagnetic
layers. In other words, it may be regarded that the
lower layers form a large source of noise, but this
embodiment can improve the SNR because the signals
from the lower layers are cancelled such that most
of the signals and thus also noise come from the
upper layers.

A third embodiment of the magnetic
recording medium according to the present invention
is based on the above findings.

In other words, in this third embodiment,
the magnetic recording medium comprises a substrate,
an underlayer disposed above the substrate, and a
magnetic layer structure including at least a bottom
ferromagnetic layer provided on the underlayer and
having a remanent magnetization and thickness
product Mriδi, and a top ferromagnetic layer
disposed above the bottom ferromagnetic layer and
having a remanent magnetization and thickness
product Mrjδj, wherein a relationship Mrδ≒Σ(Mriδi-
Mrjδj) is satisfied, where Mrδ denotes a total
remanent magnetization and thickness product of the
magnetic layer structure, so that magnetization
directions of adjacent ferromagnetic layers in the
magnetic layer structure are closely antiparallel.
δ, δi, and δj may be regarded as effective
thicknesses.

The magnetic recording medium may further
comprise a non-magnetic coupling layer interposed
between two adjacent ferromagnetic layers of the
magnetic layer structure, so that antiparallel
magnetic interaction is induced thereby. This non-magnetic
coupling layer may be made essentially of
Ru with a thickness of approximately 0.4 to 1.0 nm.
This non-magnetic coupling layer may be made of a
material selected from a group of Ru, Rh, Ir, Cu, Cr
and alloys thereof.

In the magnetic recording medium, each of
the ferromagnetic layers of the magnetic layer
structure may be made of a material selected from a
group of Co, Fe, Ni, CoCrTa, CoCrPt and CoCrPt-M,
where M=B, Cu, Mo, Nb, Ta, W and alloys thereof. In
addition, at least one of the ferromagnetic layers
of the magnetic layer structure may made up of a
plurality of ferromagnetic layers which are in
contact with each other and ferromagnetically
coupled. The Mrjδj of the top ferromagnetic layer
may be largest among products of remanent
magnetization and thickness of other ferromagnetic
layers of the magnetic layer structure. Furthermore,
the ferromagnetic layers of the magnetic layer
structure may have mutually different compositions.

According to this third embodiment of the
magnetic recording medium, the thermal stability and
the isolated wave medium SNR Siso/Nm respectively
are larger than those obtained by a magnetic
recording medium with similar Mrδ but having single
or multiple magnetic layers of closely parallel
magnetizations. Further, the PW50 value is smaller
than that obtained by a magnetic recording medium
having a similar total magnetic layer thickness.

In addition, the dHc/dT value obtained in
this third embodiment of the magnetic recording
medium is smaller than that of the magnetic
recording medium with similar Mδ but having single
or multiple magnetic layers of closely parallel
magnetizations.

Furthermore, it was confirmed from data
such as those shown in FIGS. 16 and 17 that the
ferromagnetic coupling obtained in this third
embodiment of the magnetic recording medium is
sufficiently strong and closely antiparallel in a
temperature range of approximately -10°C to 150°C.

Of course, the embodiment of the magnetic
storage apparatus described above may also use one
or more magnetic recording media according to the
third embodiment of the magnetic recording medium
described above.

Next, a description will be given of an
embodiment of a recording method according to the
present invention. This embodiment of the recording
method uses any one of the embodiments of the
magnetic recording medium described above, to
magnetically record information on the magnetic
recording medium in the embodiment of the magnetic
storage apparatus described above.

More particularly, the method of
magnetically recording information on the magnetic
recording medium, comprises a step of switching
magnetization direction of at least one of the
ferromagnetic layers which form the magnetic layer
structure of the magnetic recording medium and have
antiparallel magnetization directions, as in the
third embodiment of the magnetic recording medium.
According to this embodiment, it is possible to make
a high-density recording with improved thermal
stability.

Next, a description will be given of an
embodiment of a method of producing the magnetic
recording medium according to the present invention.

When producing any one of the embodiments
of the magnetic recording medium described above,
the crystal properties and crystal orientation of
the layers forming the magnetic recording medium
must be appropriately controlled. The non-magnetic
coupling layer in particular is extremely thin
compared to the other layers such as the underlayer,
and it is desirable that such a thin non-magnetic
coupling layer is uniformly grown. Furthermore, in
order to achieve the proper ferromagnetic coupling,
the interfaces between two adjacent layers must be
extremely clean and include no notable abnormalities.

Accordingly, in this embodiment of the
medium producing method, the layers of the magnetic
recording medium are formed sequentially (or
continuously), preferably by sequential (or
continuous) sputtering, since the sputtering enables
an extremely thin and uniform layer to be grown as
compared to other layer formation techniques. In
addition, it is possible to minimize contamination
between the adjacent layers by employing the
sequential (or continuous) sputtering.

Furthermore, even in the case of the
sputtering, it is difficult to guarantee uniform
growth of a thin film having a thickness on the
order of approximately 1 nm or less. Based on
experiments conducted by the present inventors, the
sputtering rate is preferably set to 0.35 nm/s or
less in order to guarantee the uniformity of the
grown thin film.

Moreover, when the gas pressure during the
sputtering is too high, the layers and the interface
between the adjacent layers are easily contaminated.
On the other hand, when the gas pressure during the
sputtering is too low, unstable plasma causes non-uniform
growth of the thin film. According to
experiments conducted by the present inventors, the
gas pressure during the sputtering is preferably set
on the order of approximately 5 mTorr.

In addition, the substrate temperature
during the sputtering also needs to be optimized. A
substrate temperature which is too high may cause
the substrate to warp, thereby causing non-uniform
growth of particularly the thin non-magnetic
coupling layer. On the other hand, a substrate
temperature which is too low may cause layers having
unsatisfactory crystal properties to be grown.
According to experiments conducted by the present
inventors, the substrate temperature prior to the
sputtering is set in a range of approximately 100°C
to 300°C.

FIG. 20 is a diagram showing the general
construction of a magnetic recording medium
producing apparatus which is used in this embodiment
of the medium producing method. The apparatus shown
in FIG. 20 generally includes a loading and
unloading unit 50, a heating chamber 51, and a
plurality of sputtering chambers 52-1 through 52-n,
where n depends on the layer structure of the
magnetic recording medium which is produced. The
last sputtering chamber 52-n connects to the loading
and unloading unit 50 so as to enable unloading of
the produced magnetic recording medium. For the
sake of convenience, it is assumed that n=9.

First, a substrate is loaded into the
loading and unloading unit 50 and heated to a
substrate temperature in a range of approximately
100°C to 300°C within the heating chamber 51. Then,
sequential (or continuous) DC sputtering is
successively carried out in the sputtering chambers
52-1 through 52-9 to form on the substrate a NiAl
layer which is 40 nm thick, a CrMo underlayer which
is 20 nm thick, a CoCr intermediate layer which is
1.5 nm thick, a CoCrPtB ferromagnetic layer which is
4 nm thick, a Ru non-magnetic coupling layer which
is 0.8 nm thick, a CoCrPtB ferromagnetic layer which
is 4 nm thick, a Ru non-magnetic coupling layer
which is 0.8 nm thick, a CoCrPtB magnetic layer, and
a C protection layer.

The Ar gas pressure in the sputtering
chambers 52-1 through 52-9 are set to approximately
5 mTorr. In addition, the sputtering rate is set
approximately 0.35 nm/s or less and slower in the
sputtering chambers 52-5 and 52-7 than in the other
sputtering chambers. The slower sputtering rate can
be achieved by increasing the distance between the
target and the substrate by increasing the
separation of the cathodes, as shown for the
sputtering chamber 52-5 and 52-7.

FIG. 21 is a diagram showing the
dependence of isolated wave output on effective
magnetic layer thickness. In FIG. 21, the ordinate
indicates the isolated wave output (µVpp), and the
abscissa indicates the effective magnetic layer
thickness (nm). The data shown in FIG. 21 was
obtained by writing signals on the produced magnetic
recording medium and reading the written signal
using a GMR head. It was confirmed that the
isolated wave output is proportional to the
effective magnetic layer thickness, verifying the
antiparallel ferromagnetic coupling of the magnetic
layer structure.

FIG. 22 is a diagram showing the
temperature dependence of high-frequency SNR. In
FIG. 22, the ordinate indicates the high-frequency
SNR (dB), and the abscissa indicates the substrate
temperature (°C) during the sputtering. It was
confirmed that good properties of the grown layers
are obtained, preferably when the substrate
temperature is set in a range of approximately 100°C
to 300°C.

FIG. 23 is a diagram showing a relation
ship of the isolated wave medium SNR Siso/Nm and the
sputtering rate of Ru. In FIG. 23, the ordinate
indicates the isolated wave medium SNR Siso/Nm (dB,
relative value), and the abscissa indicates the
sputtering rate (nm/s). The data shown in FIG. 23
were obtained to confirm whether or not the
ferromagnetic layer and the magnetic layer
respectively provided under and above the Ru layer
would form a norm magnetic coupling. For the sake
of convenience, the data shown in FIG. 23 were
obtained for a case where the Ru layer is formed to
a thickness of 1.4 nm on the CCPB ferromagnetic
layer, and the CCPB magnetic layer is formed on the
Ru layer.

In FIG. 23, the isolated wave medium SNR
Siso/Nm is indicated by a relative value with
respect to a comparison model medium having no Ru
layer. It may be seen from FIG. 23 that the
isolated wave medium SNR Siso/Nm deteriorates as the
sputtering rate of Ru increases. This indicates
that the extremely thin Ru layer is not formed
uniformly at high sputtering rates. FIG. 23
indicates that the isolated wave medium SNR Siso/Nm
becomes poorer than that of the comparison model
medium having no Ru layer, particularly when the
sputtering rate of Ru becomes greater than 0.35 nm/s.
Therefore, it was confirmed that the sputtering rate
of Ru should be set to 0.35 nm/s or less in order to
produce a magnetic recording medium having the high
performance described above.

Next, a description will be given of a
fourth embodiment of the magnetic recording medium
according to the present invention. In this fourth
embodiment, a magnetic bonding layer is further
provided at least between the ferromagnetic layer
and the non-magnetic coupling layer or, between the
magnetic layer and the non-magnetic coupling layer
of the first or second embodiment described above.
In this fourth embodiment, the magnetic bonding
layer is additionally provided to increase the
exchange coupling effect, so as to further improve
the thermal stability.

FIG. 24 is a cross sectional view showing
an important part of the fourth embodiment of the
magnetic recording medium according to the present
invention.

The magnetic recording medium includes a
non-magnetic substrate 101, a seed layer 102, an
underlayer 103, a non-magnetic intermediate layer
104, a ferromagnetic layer 105, a lower magnetic
bonding layer 106, a non-magnetic coupling layer 107,
an upper magnetic bonding layer 108, a magnetic
layer 109, a protection layer 110, and a lubricant
layer 111 which are stacked in this order as shown
in FIG. 24.

Although two magnetic bonding layers are
provided in this embodiment, it is possible to
provide only one of the upper and lower magnetic
bonding layers 108 and 106. When both the upper and
lower magnetic bonding layers 108 and 106 are
provided, the exchange coupling effects of the upper
and lower magnetic bonding layers 108 and 106 are
set so as to be greater than the exchange coupling
effects of the magnetic layer 109 and the
ferromagnetic layer 105. By setting the exchange
coupling effects of the upper and lower magnetic
bonding layers 108 and 106 in this manner, the
exchange coupling strength is increased above and
below the non-magnetic coupling layer 107, so that
the thermal stability of the magnetic recording
medium is improved.

If only one magnetic bonding layer is
provided, the exchange coupling strength is
increased between the lower magnetic bonding layer
106 and the magnetic layer 109 or, between the upper
magnetic bonding layer 108 and the ferromagnetic
layer 105, thereby improving the thermal stability
of the magnetic recording medium.

For example, the non-magnetic substrate
101 is made of Al, Al alloy or glass. The non-magnetic
substrate 101 may or may not be
mechanically textured.

The seed layer 102 is made of NiP, for
example, especially in the case where the non-magnetic
substrate 101 is made of Al or Al alloy.
The NiP seed layer 102 may or may not be oxidized
and may or may not be mechanically textured.
Especially in the case where the non-magnetic
substrate 101 is made of glass, the seed layer 102
is made of an alloy having the B2 structure and
selected from a group of materials including NiAl
and FeAl, for example. The seed layer 102 is
provided to promote a (001) or a (112) texture of
the underlayer 103.

In a case where the magnetic recording
medium is a magnetic disk, the mechanical texturing
provided on the non-magnetic substrate 101 or the
NiP seed layer 102 is made in a circumferential
direction of the disk, that is, in a direction in
which tracks of the disk extend.

The non-magnetic intermediate layer 104 is
provided to further promote epitaxy, narrow the
grain distribution width of the magnetic layer 109,
and orient the anisotropy axes of the magnetic layer
109 along a plane parallel to the recording surface
of the magnetic recording medium. This non-magnetic
intermediate layer 104 is made of a hcp structure
alloy such as CoCr-M, where M = B, Mo, Nb, Ta, W, Cu
or alloys thereof, and has a thickness in a range of
1 to 5 nm.

The ferromagnetic layer 105 is made of Co,
Ni, Fe, Co-based alloys, Ni-based alloys, Fe-based
alloys or the like. In other words, Co-based alloys
such as CoCrTa, CoCrPt and CoCrPt-M, where M = B, Mo,
Nb, Ta, W, Cu or alloys thereof may be used for the
ferromagnetic layer 105. For example, the
ferromagnetic layer 105 has a thickness in a range
of approximately 2 to 10 nm.

The lower magnetic bonding layer 106 is
made of Co, Fe, Ni-based alloys, Co-based alloys,
Fe-based alloys or the like. In other words, Co-based
alloys such as CoCrTa, CoCrPt and CoCrPt-M may
be used for the lower magnetic bonding layer 106,
where M = B, Mo, Nb, Ta, W, Cu or alloys thereof.
The Co concentration or Fe concentration of the
lower magnetic bonding layer 106 is desirably higher
than the Co concentration or Fe concentration of the
ferromagnetic layer 105. The lower magnetic bonding
layer 106 has a thickness in a range of
approximately 1 to 5 nm.

In a case where Co or Fe is used for the
ferromagnetic layer 105, it is possible to omit the
lower magnetic bonding layer 106. On the other hand,
when providing the lower magnetic bonding layer 106,
Fe or Co is used in reverse to the ferromagnetic
layer 105.

In other words, the Co or Fe concentration
of the lower magnetic bonding layer 106 (and the
upper magnetic bonding layer 108 which will be
described later) is preferably higher than the Co or
Fe concentrations of the ferromagnetic layer 105 and
the magnetic layer 109. If Co or Fe is used for the
ferromagnetic layer 105 or the magnetic layer 109,
the lower magnetic bonding layer 106 may be omitted.
When providing the magnetic bonding layer 106, the
material used for the magnetic bonding layer 106 is
preferably in reverse to that used for the
ferromagnetic layer 105 or the magnetic layer 109,
that is, Fe or Co is used for the magnetic bonding
layer 106.

When Ru, Rh, Ir, Cu, Ru-based alloys, Rh-based
alloys, Ir-based alloys or Cu-based alloys are
used for the non-magnetic coupling layer 107, Co,
Co-based alloys or NiFe is desirably used for the
magnetic bonding layer 106. In addition, the
magnetic bonding layer 106 is desirably made of Fe
or Fe-based alloys when the non-magnetic coupling
layer 107 is made of Cr or Cr-based alloys.

For example, when the non-magnetic
coupling layer 107 is made of Ru, the thickness of
the non-magnetic coupling layer 107 is set in a
range of approximately 0.4 to 1.0 nm, and preferably
to approximately 0.8 nm. By setting the thickness
of the non-magnetic coupling layer 107 to such a
range, the magnetizations of the ferromagnetic layer
105 and the magnetic layer 109 become antiparallel.

In other words, the magnetization
directions of the ferromagnetic layer 105 and the
magnetic layer 109 may be mutually antiparallel or
mutually parallel.

In the case of the mutually antiparallel
magnetization directions, the non-magnetic coupling
layer 107 desirably has a thickness in a range of
approximately 0.4 to 1.0 nm when made of a material
selected from a group of Ru, Rh, Ir, Cr, Ru-based
alloys, Rh-based alloys, Ir-based alloys and Cr-based
alloys, and has a thickness in a range of
approximately 1.5 to 2.1 nm when made of a material
selected from a group of Cu and Cu-based alloys.

On the other hand, in the case of mutually
parallel magnetization directions, the non-magnetic
coupling layer 107 desirably has a thickness in a
range of approximately 0.2 to 0.4 nm and 1.0 to 1.7
nm when made of a material selected from a group of
Ru, Rh, Ir, Cu, Ru-based alloys, Rh-based alloys,
Ir-based alloys and Cu-based alloys, and has a
thickness in a range of approximately 1.0 to 1.4 nm
and 2.6 to 3.0 nm when made of a material selected
from a group of Cr and Cr-based alloys.

The upper magnetic bonding layer 108 is
made of a material similar to that of the lower
magnetic bonding layer 106. In addition, the Co
concentration or Fe concentration of the upper
magnetic bonding layer 108 is preferably higher than
the Co concentration or Fe concentration of the
magnetic layer 109. The upper magnetic bonding
layer 108 has a thickness in a range of
approximately 1 to 5 nm. In a case where Co or Fe
is used for the magnetic layer 109, it is possible
to omit the upper magnetic bonding layer 108. On
the other hand, when providing the upper magnetic
bonding layer 108, Fe or Co is used in reverse to
the magnetic layer 109.

The upper and lower magnetic bonding
layers 108 and 106 may be made of a material
different from those of the ferromagnetic layer 105
and the magnetic layer 109. In this case, a
different material may have the same material
composition but with a different material content
ratio.

The exchange coupling between the upper
and lower magnetic bonding layers 108 and 106 is
desirably larger than the exchange coupling between
the magnetic layer 109 and the ferromagnetic layer
105.

When using Ru, Rh, Ir, Cu, Ru-based alloys,
Rh-based alloys, Ir-based alloys or Cu-based alloys
for the non-magnetic coupling layer 107, it is
desirable to use Co, Co-based alloys or NiFe for the
upper and lower magnetic bonding layers 108 and 106.
On the other hand, when using Cr or Cr-based alloys
for the non-magnetic coupling layer 107, it is
desirable to use Fe or Fe-based alloys for the upper
and lower magnetic bonding layers 108 and 106.

The ferromagnetic layer 105 and the non-magnetic
coupling layer 107 form the basic exchange
layer structure. The upper and lower magnetic
bonding layers 108 and 106 which are provided above
and below the non-magnetic coupling layer 107 have
the function of increasing the exchange coupling
effects of the exchange layer structure.

The magnetic layer 109 is made of a
material selected from a group of Co, Ni, Fe, Ni-based
alloys, Fe-based alloys and Co-based alloys
such as CoCrTa, CoCrPt and CoCrPt-M, where M = B, Mo,
Nb, Ta, W, Cu or alloys thereof. The magnetic layer
109 has a thickness in a range of 5 to 30 nm. Of
course, the magnetic layer 109 is not limited to a
single-layer structure, and a multi-layer structure
may be used for the magnetic layer 109.

The protection layer 110 and the lubricant
layer 111 are similar to those of the first and
second embodiments described above. Obviously, the
layer structure under the exchange layer structure
is not limited to that shown in FIG. 24. For
example, the underlayer 103 may be made of Cr or Cr-based
alloys and formed to a thickness in a range of
5 to 40 nm on the substrate 101, and the exchange
layer structure may be provided on this underlayer
103.

In this fourth embodiment, the magnetic
recording medium may further comprise a substrate
and an underlayer provided above the substrate, such
that the exchange layer structure is provided above
the underlayer. Furthermore, the magnetic recording
medium may further comprise a non-magnetic
intermediate layer provided between the underlayer
and the exchange layer structure, where the non-magnetic
intermediate layer is made of a CoCr-M
alloy having a hcp structure and a thickness of
approximately 1 to 5 nm, where M = B, Mo, Nb, Ta, W,
Cu or alloys thereof. Moreover, the magnetic
recording medium may further comprise a seed layer
provided between the substrate and the underlayer.
The seed layer may be made of NiP which may or may
not be mechanically textured, and may or may not be
oxidized. In addition, the seed layer may be made
of an alloy having a B2 structure such as NiAl and
FeAl.

The magnetic recording medium may further
comprise at least a first exchange layer structure
and a second exchange layer structure provided
between the first exchange layer structure and the
magnetic layer, where the second exchange layer
structure has a ferromagnetic layer with a magnetic
anisotropy smaller than that of a ferromagnetic
layer of the first exchange layer structure, and the
first and second exchange layer structures have
ferromagnetic layers with magnetization directions
which are mutually antiparallel.

The magnetic recording medium may further
comprise at least a first exchange layer structure
and a second exchange layer structure provided
between the first exchange layer structure and the
magnetic layer, where the second exchange layer
structure has a ferromagnetic layer with a remanent
magnetization and thickness product smaller than
that of a ferromagnetic layer of the first exchange
layer structure, and the first and second exchange
layer structures have ferromagnetic layers with
magnetization directions which are mutually
antiparallel.

FIG. 25 is a diagram showing the in-plane
characteristic of two CoCr-based alloy layers
separated by Ru, for a case where a seed layer, an
underlayer, a non-magnetic intermediate layer, a
ferromagnetic layer, a Ru non-magnetic coupling
layer, a CoCr-based alloy magnetic layer are
successively stacked in this order on a glass
substrate.

It is assumed that the same CoCr-based
alloy is used for the ferromagnetic layer and the
magnetic layer. In FIG. 25, two loops are shown for
different concentrations of Co and Cr, but the layer
structure and compositions other than Co and Cr are
the same for the two loops. In FIG. 25, the
ordinate indicates the magnetization (emu/cc), and
the abscissa indicates the magnetic field (Oe).

As may be seen from FIG. 25, a shift
occurs in both the two loops in the vicinity of the
ordinate, verifying the generation of the anti-ferromagnetic
coupling. Furthermore, it may be seen
from FIG. 25 that the loop indicated by the dashed
line for the higher concentration of Co (Co-rich)
has the larger coercivity. Even in the case of the
conventional magnetic recording medium having no
exchange layer structure, the coercivity is improved
by approximately 400 Oe for the magnetic layer with
the high Co concentration as compared to the
magnetic layer with the low Co concentration. Since
the loop shift occurs when a sum of the externally
applied magnetic field and the magnetic field caused
by the anti-ferromagnetic coupling introduced
between the magnetic layer and the ferromagnetic
layer becomes equal to the coercivity, a difference
between the loop shift position and the coercivity
becomes the strength of the anti-ferromagnetic
exchange coupling. In FIG. 25, the loop shifts
occur approximately at the same magnetic field
position, but it may be seen that the exchange
coupling is larger for the Co-rich case indicated by
the dashed line due to a difference in coercivities
between the two cases. In addition, the aspect
ratio of the Co-rich loop is better than the other
loop.

Therefore, by using a Co-rich alloy for
the magnetic bonding layer, it is possible to
increase the exchange coupling effect and realize a
magnetic recording medium having a further improved
thermal stability.

In the magnetic recording medium having
the exchange layer structure such as that of the
first embodiment shown in FIG. 1, when Ru is used
for the non-magnetic coupling layer 8 and a CoCr-based
alloy is used for the magnetic layer 9, both
of these layers 8 and 9 have the hcp structure. In
order to increase both the coercivity and resolution
of the magnetic recording medium, it is desirable
that the c-axis of the hcp structure is parallel
with respect to the surface of the substrate 1. In
a case where a CoCr-based alloy is used for the
ferromagnetic layer 7, the ferromagnetic layer 7 is
grown epitaxially on the non-magnetic intermediate
layer 6 which is made of an alloy having the hcp
structure and oriented on the (002) face, and thus,
the in-plane orientation of the c-axis of the
ferromagnetic layer 7 is very satisfactory.

On the other hand, the Ru which is used
for the non-magnetic coupling layer 8 has the hcp
structure, similarly as in the case of the CoCr-based
alloy, but the lattice constant of Ru is
approximately 5% larger than the lattice constant of
the CoCr-based alloy. For this reason, the
epitaxial growth may be slightly obstructed due to
the lattice mismatch between the ferromagnetic layer
7 and the non-magnetic coupling layer 8 or, between
the non-magnetic coupling layer 8 and the magnetic
layer 9. If the epitaxial growth is slightly
obstructed due to the lattice mismatch, the
coercivity of the magnetic recording medium
decreases, and the in-plane orientation of the c-axis
of the CoCr-based alloy becomes unstable.

Next, a description will be given of an
embodiment which can improve the epitaxial growth
between the Ru and the CoCr-based alloy,
simultaneously increase the coercivity of the
magnetic recording medium and improve the in-plane
orientation of the c-axis of the CoCr-based alloy,
and improve mainly the recording resolution
characteristic of the magnetic recording medium.

FIG. 26 is a cross sectional view showing
an important part of a fifth embodiment of the
magnetic recording medium according to the present
invention.

The magnetic recording medium includes a
non-magnetic substrate 216, a seed layer 217, an
underlayer 218 made of a Cr-based alloy, a non-magnetic
intermediate layer 219, a ferromagnetic
layer 220, a non-magnetic coupling layer 221, a
magnetic layer 222, a protection layer 223, and a
lubricant layer 224 which are stacked in this order
as shown in FIG. 26.

The non-magnetic substrate 216 is made of
an Al alloy or glass, for example. The non-magnetic
substrate 216 may or may not be mechanically
textured. The seed layer 217 is made of NiP which
is plated in a case where the non-magnetic substrate
216 is made of the Al alloy. The NiP seed layer 217
may or may not be mechanically textured. On the
other hand, in a case where the non-magnetic
substrate 216 is made of glass, the seed layer 217
is made of an intermetallic compound material having
the B2 structure, such as NiAl and FeAl.

The non-magnetic intermediate layer 219 is
provided to promote the epitaxial growth of the
magnetic layer 222, reduction of the grain size
distribution with of the magnetic layer 222, and the
anisotropic axis (c-axis, axis of easy
magnetization) orientation of the magnetic layer 222
along a plane parallel to the recording surface of
the magnetic recording medium. The non-magnetic
intermediate layer 219 is made of an alloy having
the hcp structure, such as CoCr-M1, and has a
thickness in a range of approximately 1 to 5 nm,
where M1 = B, Mo, Bn, Ta, W or alloys thereof.

The ferromagnetic layer 220 is made of a
material selected from a group of Co, Ni, Fe, Co-based
alloys, Ni-based alloys, Fe-based alloys and
the like. In other words, Co-based alloys including
CoCrTa, CoCrPt and CoCrPt-M2 may be used for the
ferromagnetic layer 220, where M2 = B, Mo, Nb, Ta, W
or alloys thereof. The ferromagnetic layer 220 has
a thickness in a range of approximately 2 to 10 nm.

The non-magnetic coupling layer 221 is
made of an alloy having the hcp structure, such as
Ru-M3, where M3 = Co, Cr, Fe, Ni, Mn or alloys
thereof. For example, the non-magnetic coupling
layer 221 has a thickness in a range of
approximately 0.4 to 1.0 nm, and preferably in a
range of approximately 0.6 to 0.8 nm. By setting
the thickness of the non-magnetic coupling layer 221
within such a range, the magnetizations of the
ferromagnetic layer 220 and the magnetic layer 222
become antiparallel. Accordingly, the ferromagnetic
layer 220 and the non-magnetic coupling layer 219
form an exchange layer structure.

The magnetic layer 222 is made of a
material selected from a group of Co and Co-based
alloys including CoCrTa, CoCrPt, and CoCrPt-M4,
where M4 = B, Mo, Nb, Ta, W or alloys thereof. The
magnetic layer 222 has a thickness in a range of
approximately 5 to 30 nm. Of course, the magnetic
layer 222 is not limited to a single-layer structure,
and a multi-layer structure may be used for the
magnetic layer 222.

The protection layer 223 is made of C or
diamond-like C (DLC). In addition, the lubricant
layer 224 is made of an organic lubricant agent, in
order to enable the magnetic recording medium to be
used with a magnetic transducer such as a spin valve
head. The protection layer 223 and the lubricant
layer 224 form a protection layer structure of the
magnetic recording medium.

As described above, the non-magnetic
coupling layer 221 is made of an alloy Ru-M3, where
M3 = Co, Cr, Fe, Ni, Mn or alloys thereof. In this
embodiment, an amount of the element M3 added to the
Ru is set within the following composition ranges so
as to maintain a stable hcp structure. In the
following composition ranges, the numerical values
following the brackets respectively indicate the
amount in atomic percent (at%).

Ru-Co(0 to 50 at%)

Ru-Cr(0 to 50 at%)

Ru-Fe(0 to 60 at%)

Ru-Ni(0 to 10 at%)

Ru-Mn(0 to 50 at%)

FIG. 27 is a diagram showing a
magnetization curve which is obtained when pure Ru
is used for the non-magnetic coupling layer 221 of
the magnetic recording medium shown in FIG. 26. In
FIG. 27, the ordinate indicates the magnetization M
(arbitrary units), and the abscissa indicates the
magnetic field H (kOe). The magnetization curve
shown in FIG. 27 was measured by a vibrating sample
type magnetometer while applying a magnetic field
parallel to the sample surface, that is, parallel to
the recording surface of the magnetic recording
medium. The magnetization curve has a constricted
portion because of the existence of a region where
the ferromagnetic layer 220 and the magnetic layer
222 form an antiparallel coupling.

In addition, a magnetization curve which
is obtained when a Ru-M3 alloy is used for the non-magnetic
coupling layer 221 was also measured
similarly to the above. In the case where the Ru-M3
alloy is used for the non-magnetic coupling layer
221, it was also confirmed that a constricted
portion is formed in the magnetization curve,
similarly as in the case shown in FIG. 27, due to
the existence of the region where the ferromagnetic
layer 220 and the magnetic layer 222 form the
antiparallel coupling.

In the first and fourth quadrants in FIG.
27, a linear portion of the magnetization curve on
the higher magnetic field side of the constricted
portion is extrapolated to the magnetic field axis,
and the intersection with the magnetic field axis is
defined as an in-plane coercivity Hc//.

FIG. 28 is a diagram showing a
magnetization curve which is measured by a vertical
Kerr looper (or loop) while applying a magnetic
field in a perpendicular direction with respect to
the sample surface, with respect to the magnetic
recording medium for which the data shown in FIG. 27
were measured. In FIG. 28, the ordinate indicates
the Kerr rotation (degrees), and the abscissa
indicates the perpendicular magnetic field (Oe). A
definition of a perpendicular coercivity Hc⊥ is
shown in FIG. 28.

The extent of the in-plane orientation of
the axis of easy magnetization of the magnetic layer
222 can be evaluated by a ratio (Hc⊥)/(Hc//). The
smaller this ratio (Hc⊥)/(Hc//), the better the in-plane
orientation of the magnetic layer 222.

Measured results of the in-plane
coercivity Hc// and the ratio (Hc⊥)/(Hc//) for
various materials used for the non-magnetic coupling
layer 221 are shown in the following. In the
following, the in-plane coercivity Hc// of the
various materials is indicated by a relative value
relative to the in-plane coercivity Hc// = 1 for the
case where pure Ru is used for the non-magnetic
coupling layer 221.

Non-Magnetic Coupling Layer 221

Hc// (Relative Value)

(Hc⊥)/(Hc//)

Ru

1

0.33

Ru-Co(20at%)

1.10

0.23

Ru-Cr(20at%)

1.05

0.25

Ru-Fe(20at%)

1.07

0.28

Ru-Mn(20at%)

0.96

0.30

Ru-Ni(10at%)

0.94

0.30

Therefore, it was confirmed that the
ratios (Hc⊥)/(Hc//) for the cases where the Ru-M3
alloys are used for the non-magnetic coupling layer
221 are improved according to this embodiment, as
compared to the case where pure Ru is used for the
non-magnetic coupling layer 221. As a result, it
was confirmed that the recording resolution is
improved by approximately 1.5 to 2.5 % by the
improved in-plane orientation of the magnetic layer
222.

A lattice mismatch of the intervals of the
(002) faces of the hcp structure of the Ru used for
the non-magnetic coupling layer 221 with respect to
the magnetic layer 222 and the ferromagnetic layer
220 respectively disposed above and below the non-magnetic
coupling layer 221 is normally
approximately 5 % at the maximum and approximately
8 % in a worst case. But by the addition of the
element M3 to the Ru, it was confirmed that the
lattice mismatch can be reduced to approximately 6 %
or less, and preferably approximately 2 % or less.
Furthermore, the element M3 added to the Ru is
preferably Co, Cr, Fe, Ni, Mn or alloys thereof, but
it is of course possible to adjust the lattice
mismatch by adding to the Ru a material selected
from a group of Ir, Mo, Nb, Pt, Rh, Ta, Ti, V, W and
alloys thereof.

Of course, this embodiment may be applied
similarly to the construction of the second
embodiment of the magnetic recording medium
described above.

Next, the prior art will be briefly
summarized before explaining further features of the
present invention in conjunction with FIGS. 29
through 32.

Due to the development of the information
processing technology, there are increased demands
for high-density magnetic recording media.
Characteristics required of the magnetic recording
media to satisfy such demands include low noise,
high coercivity, high remanence magnetization, and
high resolution.

Conventionally, various measures have been
proposed to reduce the noise in the magnetic
recording media. A general magnetic recording
medium basically includes a non-magnetic substrate
made of Al or the like, and an underlayer, a
magnetic recording layer, a protection layer and a
lubricant layer which are stacked in this order on
the substrate. For example, with respect to the
underlayer, functions such as promoting the in-plane
orientation of the magnetic recording layer and
increasing the remanence magnetization and thermal
stability of written bits are demanded, in order to
improve the magnetization characteristic of the
magnetic recording layer. When a suitable
underlayer is used, it is possible to reduce the
thickness of the magnetic recording layer or, to
reduce the size of the magnetic grains and the grain
size distribution width of the magnetic recording
layer, thereby enabling noise reduction.

In addition, there are proposals to
increase the resolution by reducing the thickness of
the magnetic layer or, to reduce the transition
width between the written bits. There are also
proposals to promote the Cr segregation of the CoCr-based
alloy which forms the magnetic recording layer,
so as to reduce the exchange coupling among the
magnetic grains. These proposals have been made to
reduce noise from various aspects.

As one technique which is effective in
reducing noise, there is a proposal to employ a
multi-layer structure for the magnetic recording
layer, by separating the magnetic recording layer
into upper and lower portions by a separation layer.
By employing the multi-layer structure with the
separation layer for the magnetic recording layer,
it is possible to separate the magnetic coupling of
the magnetic layers, and to positively reduce the
noise.

However, in the magnetic recording layer
having the multi-layer structure, a non-magnetic
material such as CoCr-based alloys and Cr-based
alloys are used for the separation layer. However,
such non-magnetic materials easily mix with the
magnetic layers which are provide above and below,
thereby deteriorating the magnetic characteristics
of the magnetic recording layer, and there was a
problem in that the reproduced output obtained from
such a magnetic recording medium is deteriorated
thereby. Furthermore, the non-magnetic materials
such as the CoCr-based alloys and Cr-based alloys
virtually separate completely the magnetic coupling
of the magnetic layers provided above and below.
Hence, although such non-magnetic materials are
preferable from the point of reducing the noise, the
magnetic layers provided above and below become
thermally unstable as a result. Consequently, in
the case of a magnetic recording medium such as a
magnetic disk which may be used under a relatively
high temperature environment, there was a problem in
that the detection sensitivity of written bits
deteriorates due to the provision of such a
separation layer.

Next, a description will be given of a
sixth embodiment of the magnetic recording medium
according to the present invention, which has a
magnetic recording layer with a multi-layer
structure, wherein the noise can be reduced while
maintaining the desired thermal stability and
magnetic characteristics.

In the first and second embodiments, a
magnetic recording medium comprising at least one
exchange layer structure, and a magnetic layer
provided on the exchange layer structure, wherein
the exchange layer structure includes a
ferromagnetic layer and a non-magnetic coupling
layer provided on the ferromagnetic layer and under
the magnetic layer, and the magnetization directions
of the ferromagnetic layer and the magnetic layer
are mutually antiparallel.

In the first and second embodiments, for a
particular Ru or Ir layer thickness between two
ferromagnetic layers, the magnetizations can be made
parallel or antiparallel. For example, for a
structure made up of two ferromagnetic layers of
different thickness with antiparallel magnetizations,
the effective grain size of a magnetic recording
medium can be increased without significantly
affecting the resolution. A signal amplitude
reproduced from such a magnetic recording medium is
reduced due to the opposite magnetizations, but this
can be rectified by adding another layer of
appropriate thickness and magnetization direction,
under the laminated magnetic layer structure, to
thereby cancel the effect of one of the layers. As
a result, it is possible to increase the signal
amplitude reproduced from the magnetic recording
medium, and to also increase the effective grain
volume. Thermally stable written bits can therefore
be realized.

The first and second embodiments increase
the thermal stability of written bits by exchange
coupling the magnetic layer to another ferromagnetic
layer with an opposite magnetization or, by a
laminated ferrimagnetic structure. The
ferromagnetic layer or the laminated ferrimagnetic
structure is made up of exchange-decoupled grains as
the magnetic layer. In other words, the first and
second embodiments use an exchange pinning
ferromagnetic layer or a ferrimagnetic multilayer to
improve the thermal stability performance of the
magnetic recording medium.

In the sixth embodiment described
hereunder, the above findings associated with the
first and second embodiments are effectively
utilized, by noting that the magnetization
directions of the two ferromagnetic layers become
mutually parallel when the layer made of Ru or the
like provided between the two ferromagnetic layer
has a specific thickness. Accordingly, the sixth
embodiment also employs a layer structure similar to
the basic layer structure of the first and second
embodiments. The magnetic layer, the non-magnetic
coupling layer and the ferromagnetic layer of the
first and second embodiments respectively correspond
to the (first) magnetic recording layer, the non-magnetic
separation layer and the (second) magnetic
recording layer of the sixth embodiment, but the
functions of the layers of the sixth embodiment
differ from those of the first and second
embodiments.

The basic structure of the sixth
embodiment of the magnetic recording medium
according to the present invention includes a non-magnetic
substrate, an underlayer, a magnetic
recording layer, a protection layer and a lubricant
layer which are stacked in this order, similarly to
the conventional magnetic recording medium, but the
magnetic recording layer has a multi-layer structure.

FIG. 29 is a cross sectional view showing
an important part of the sixth embodiment of the
magnetic recording medium according to the present
invention. A magnetic recording medium 30 includes
a non-magnetic substrate 31, a NiP layer 32, an
underlayer 33, a non-magnetic intermediate layer 34,
a second magnetic recording layer 35, a non-magnetic
separation layer 36, a first magnetic recording
layer 37, a protection layer 38, and a lubricant
layer 39 which are stacked in this order as shown in
FIG. 29.

A magnetic recording layer of this
magnetic recording medium 30 has a 3-layer structure,
including the second magnetic recording layer 35,
the non-magnetic separation layer 36 and the first
magnetic recording layer 37. As will be described
later, the upper first magnetic recording layer 37
and the lower second magnetic recording layer 35 are
magnetically coupled via the non-magnetic separation
layer 36, and magnetization directions of the first
and second magnetic recording layers 37 and 35 are
parallel (in the same direction). When the magnetic
recording medium 30 is formed as a magnetic disk and
signals are recorded thereon by a magnetic head, the
first and second magnetic recording layers 37 and 35
which are in the above described relationship are
fixed along the recording magnetization while
maintaining the mutually parallel state. The
magnetization states of the first and second
magnetic recording layers 37 and 35 are read at the
time of the signal reproduction.

Because the first and second magnetic
recording layers 37 and 35 are separated by the non-magnetic
separation layer 36, low noise is realized
by the effects of magnetic separation. However,
unlike the conventional non-magnetic separation
layer made of CoCr-based alloys or Cr-based alloys,
a magnetic coupling which is sufficient to maintain
the mutually parallel magnetization states of the
first and second magnetic recording layers 37 and 35
exists in the case of this embodiment. As a result,
the thermal stability is improved, and the
reliability of the magnetic recording medium 30 is
improved over the conventional magnetic medium since
the magnetic characteristics desired of the magnetic
recording layer can be maintained in this embodiment
even under a relatively high temperature environment.

In the magnetic recording layer having the
multi-layer structure, two or more non-magnetic
separation layers may be provided. For example,
when providing two non-magnetic separation layers,
the magnetic recording layer is formed by first,
second and third magnetic recording layers, and a
first non-magnetic separation layer is interposed
between the first and second magnetic recording
layers, while a second non-magnetic separation layer
is interposed between the second and third magnetic
recording layers.

Returning now to the description of the
magnetic recording medium 30 shown in FIG. 29, the
non-magnetic substrate 31 is made of a non-magnetic
material such as Al, Al alloy and glass. The non-magnetic
substrate 31 may or may not be mechanically
textured.

The NiP layer 32 is made of NiP, but may
be replaced by a layer made of a material such as
NiAl and FeAl having the B2 structure. The NiP
layer 32 may or may not be mechanically textured.
In the case where the magnetic recording medium 30
is a magnetic disk and the mechanical texturing is
to be made, the non-magnetic substrate 31 and the
NiP layer 32 are mechanically textured in the
circumferential direction of the magnetic disk, that
is, in a direction in which a track on the magnetic
disk is formed.

The underlayer 33 is made of at least one
layer of a Cr-based alloy. For example, the
underlayer 33 may be made of a Cr-M alloy, where M =
Mo, Fe, Mn, Ti, V, W or alloys thereof.

The non-magnetic intermediate layer 34 is
provided to promote the epitaxial growth of the
first and second magnetic recording layers 37 and 35,
reduction of the grain size distribution width, and
orientation of the anisotropy axes (axis of easy
magnetization) of the magnetic recording layer in a
plane parallel to the recording surface of the
magnetic recording medium 30. The non-magnetic
intermediate layer 34 is made of a CoCr-M alloy
having the hcp structure, where M = B, Mn, Mo, Nb,
Ta, Ti, W or alloys thereof, and has a thickness in
a range of approximately 1 to 5 nm. It is not
essential to provide the non-magnetic intermediate
layer 34, and it is possible to omit the non-magnetic
intermediate layer 34.

The second magnetic recording layer 35 is
made of a material such as Co, Ni, Fe, Co-based
alloys, Ni-based alloys and Fe-based alloys, In
other words, Co and Co-based alloys including CoCrTa,
CoCrPt and CoCrPt-M may be used for the second
magnetic recording layer 35, where M = B, Mo, Nb, Ta,
W or alloys thereof. Preferably, the second
magnetic recording layer 35 has a thickness in a
range of approximately 2 to 10 nm.

The non-magnetic separation layer 36 is
made of a material such as Ru, Rh, Ir and alloys
thereof. When using Ru for the non-magnetic
separation layer 36, the thickness of the non-magnetic
separation layer 36 is preferably set in a
range of approximately 0.2 to 0.4 nm or
approximately 1.0 to 1.7 nm. By setting the
thickness of the non-magnetic separation layer 36 to
such a range, it is possible to set the
magnetization directions of the second magnetic
recording layer 35 and the first magnetic recording
layer 37 to become mutually parallel.

The first magnetic recording layer 37 may
be made of a material such as Co, and Co-based
alloys including CoCrTa, CoCrPt and CoCrPt-M,
similarly to the second magnetic recording layer 35,
where M = B, Mo, Nb, Ta, W or alloys thereof.
Preferably, the first magnetic recording layer 37
has a thickness in a range of approximately 5 to 30
nm. The first magnetic recording layer 37 is not
limited to a single-layer structure, and the first
magnetic recording layer 37 itself may have a multi-layer
structure.

The materials forming the first and second
magnetic recording layers 37 and 35 may be the same
or, may be mutually different.

The protection layer 38 is made of a C-based
material, for example. In addition, the
lubricant layer 39 is made of an organic lubricant
agent to enable use of the magnetic recording medium
30 with a magnetic transducer such as a spin valve
head. The protection layer 38 and the lubricant
layer 39 form a protection layer structure of the
magnetic recording medium 30.

The layer structure of sixth embodiment of
the magnetic recording medium according to the
present invention is of course not limited to that
shown in FIG. 29. For example, the underlayer 33
may be made of Cr or a Cr-based alloy, and formed to
a thickness in a range of approximately 5 to 40 nm.
The second magnetic recording layer 35 may be
provided on such an underlayer 33.

Next, a description will be given of a
method of producing the magnetic recording medium 30.
The layers formed on the substrate 31 of the
magnetic recording medium 30, from the underlayer 33
up to the protection layer 38, may be formed by
sputtering.

For example, after cleaning the NiP-plated
Al substrate 31 which has a thickness of 0.8 mm and
a diameter of 3.5 inches, a magnetron sputtering
apparatus is used to heat the Al substrate 31 to a
substrate temperature of 220°C, and the layers from
the underlayer 33 up to the protection layer 38 are
successively formed by sputtering. The NiP layer on
the Al substrate 31 may be oxidized or mechanically
textured. The sputtering is carried out at a
pressure of 5 mTorr and a constant sputtering time
of 4 seconds. The underlayer 33 is formed to a
thickness of 10 nm by sputtering a material
Cr95Mo2.5W2.5 to a thickness of 3 nm, and sputtering a
material Cr80Mo10W10 to a thickness of 7 nm. The non-magnetic
intermediate layer 34 is formed to a
thickness of 3 nm by sputtering a material Co63Cr37.
The second magnetic recording layer 35 is formed to
a thickness of 9.5 nm by sputtering a material
Co63Cr22Pt11B4. The non-magnetic separation layer 36
is formed to a thickness selected within a range of
0.2 to 0.4 nm or 1.0 to 1.7 nm by sputtering Ru.
The first magnetic recording layer 37 is formed to a
thickness of 9.5 nm by sputtering a material
Co63Cr22Pt11B4. Furthermore, the protection layer 38
is formed by sputtering a C-based material, and the
lubricant layer 39 is formed by coating an organic
lubricant agent on the protection layer 38.

FIG. 30 is a diagram showing the
relationship of a ratio Siso/Nm of the isolated wave
output (Siso) and medium noise (Nm) of this
embodiment of the magnetic recording medium 30 which
is produced as described above and the thickness of
the Ru non-magnetic separation layer 36. The ratio
Siso/Nm is a ratio of the isolated wave output Siso
at 270 kFCI and the medium noise Mm. It was
confirmed from FIG. 30 that the noise is reduced,
since the value of the ratio Siso/Nm increases as
the thickness of the non-magnetic separation layer
36 increases.

As described above, when Ru is used for
the non-magnetic separation layer 36, the
magnetization directions of the first and second
magnetic layers 37 and 35 disposed above and below
the non-magnetic separation layer 36 become mutually
parallel or mutually antiparallel depending on the
thickness of the non-magnetic separation layer 36.
In this embodiment, the magnetizations of the first
and second magnetic recording layers 37 and 35
becomes mutually parallel by setting the thickness
of the non-magnetic separation layer 36 to
approximately 0.4 nm or less or approximately 1.0 nm
or greater.

FIG. 31 is a diagram showing the
relationship of a thickness ratio of first and
second magnetic recording layers 37 and 35 and the
ratio Siso/Nm of the isolated wave output (Siso) and
medium noise (Nm), for a case where the same
material CO63Cr22Pt11B4 is used for the first and
second magnetic recording layers 37 and 35. In FIG.
31, the abscissa indicates a thickness ratio
(thickness of the first magnetic recording layer
37)/{(thickness of the first magnetic recording
layer 37)+(thickness of the second magnetic
recording layer 35)}. It was confirmed from FIG. 31
that a high ratio Siso/Nm can be obtained when the
thickness ratio is in a range of approximately 0.5
to 0.7.

FIG. 32 is a diagram showing the
relationship of the thickness ratio of the first and
second magnetic recording layers 37 and 35 and a
ratio S/Nt of an output (S) and noise (Nt). In FIG.
32, the abscissa is the same as that of FIG. 11. It
was confirmed from FIG. 32 that a high ratio S/Nt
can also be obtained when the thickness ratio is in
a range of approximately 0.5 to 0.7.

From FIGS. 31 and 32, it was confirmed
that the noise reducing effect is further improved
when a thickness ratio of the thickness of the first
magnetic recording layer 37 to the thickness of the
second magnetic recording layer 35 is in a range of
approximately 5:5 to 7:3. The thickness ratio is
desirably set close to 5:5 from the point of view of
realizing low noise, and is desirably set close to
7:3 from the point of view of realizing high
resolution.

Therefore, according to the sixth
embodiment of the magnetic recording medium, a non-magnetic
separation layer which is made of Ru or the
like and has a predetermined thickness maintains the
magnetic coupling of magnetic recording layers above
and below the non-magnetic separation layer to a
mutually parallel state. Hence, it is possible to
realize a magnetic recording medium having low noise
and desired thermal stability. Compared to the
conventional magnetic recording medium, this
magnetic recording medium has a high reliability and
is suited for high-density recording.

In addition, a magnetic storage apparatus
which uses such a magnetic recording medium can cope
with the demands of high-density recording, and
enable magnetic recording and reproduction of
information with a high sensitivity.

Next, a description will be given of a
seventh embodiment of the magnetic recording medium
according to the present invention. In this seventh
embodiment, at least one of the ferromagnetic layer
and the magnetic layer of the first or second
embodiment described above has a granular layer
structure. The granular layer structure employed in
this seventh embodiment has ferromagnetic crystal
grains uniformly distributed within a non-magnetic
base material, so as to further isolate the magnetic
grains.

In a case where both the ferromagnetic
layer and the magnetic layer have the granular layer
structure, the magnetization directions of the
granular layers can be made mutually parallel or
mutually antiparallel, similarly to the first and
second embodiments described above, by making the
non-magnetic coupling layer which is made of Ru or
the like and disposed between the granular layers to
have a predetermined thickness. As a result, it is
possible to increase the effective volume, thereby
improving the thermal stability of written bits and
reducing the medium noise.

It is not essential for both the
ferromagnetic layer and the magnetic layer to have
the granular layer structure, and the granular layer
structure may be employed for only one of the
ferromagnetic layer and the magnetic layer. When
using only one granular layer, it is desirable to
make the magnetic layer, which forms the recording
layer, to have the granular layer structure.

In this embodiment, the granular layer is
magnetically exchange coupled in an opposite
magnetization direction (antiparallel) to that of
the other granular layer or the CoCr-based magnetic
layer, so as to improve the thermal stability of the
written bits. In other words, this embodiment is
provided with a pinning structure for improving the
thermal stability performance of the magnetic
recording medium, and is also provided with the
granular layer structure for further reducing the
medium noise.

The granular layer structure refers to a
layer structure in which ferromagnetic crystal
grains are uniformly distributed within a non-magnetic
base material, as taught in a Japanese
Laid-Open Patent Application No.10-92637. A
granular medium is obtained by applying this
granular layer structure to the recording medium of
the magnetic storage apparatus. In the conventional
recording medium which uses a CoCr-based magnetic
material for the magnetic recording layer, the Co
and Cr segragations are used to promote isolation of
the magnetic grains and to reduce the noise. But in
the conventional recording medium, it was difficult
to obtain a desired isolation state of the magnetic
grains.

On the other hand, in the granular medium
according to the present invention, the
ferromagnetic crystal grains are positively isolated
by uniformly distributing the ferromagnetic crystal
grains (metal) within the base material such as SiO2
(ceramic material), and thus, it is possible to
realize a medium with extremely low noise.

FIG. 33 is a cross sectional view showing
an important part of the seventh embodiment of the
magnetic recording medium according to the present
invention.

The magnetic recording medium includes a
non-magnetic substrate 401, a first seed layer 402,
a NiP layer 403, a second seed layer 404, an
underlayer 405, a non-magnetic intermediate layer
406, a ferromagnetic layer 407, a non-magnetic
coupling layer 408, a magnetic layer 409, a
protection layer 410, and a lubricant layer 411
which are stacked in this order as shown in FIG. 33.

For example, the non-magnetic substrate
401 is made of Al, Al alloy or glass. The non-magnetic
substrate 401 may or may not be
mechanically textured.

The first seed layer 402 is made of NiP,
for example, especially in the case where the non-magnetic
substrate 401 is made of glass. The NiP
layer 403 may or may not be oxidized and may or may
not be mechanically textured. The second seed layer
404 is provided to promote a (001) or a (112)
texture of the underlayer 405 when the underlayer
405 is made of an alloy having the B2 structure,
such as NiAl and FeAl. The second seed layer 404 is
made of a material similar to that of the first seed
layer 402.

In a case where the magnetic recording
medium is a magnetic disk, the mechanical texturing
provided on the non-magnetic substrate 401 or the
NiP layer 403 is made in a circumferential direction
of the disk, that is, in a direction in which tracks
of the disk extend.

The non-magnetic intermediate layer 406 is
provided to further promote epitaxy, narrow the
grain distribution width of the magnetic layer 409,
and orient the anisotropy axes of the magnetic layer
409 along a plane parallel to the recording surface
of the magnetic recording medium. However, it is
not essential to provide this non-magnetic
intermediate layer 406. This non-magnetic
intermediate layer 406 is made of a hcp structure
alloy such as CoCr-M, where M = B, Mo, Nb, Ta, W, Cu
or alloys thereof, and has a thickness in a range of
1 to 5 nm.

The ferromagnetic layer 407 may be made of
a granular layer which is formed by uniformly
distributing ferromagnetic crystal grains into a
non-magnetic base material. In this case, the
ferromagnetic crystal grains may be made of Co, Ni,
Fe, Ni-based alloys, Fe-based alloys, or Co-based
alloys such as CoCrTa, CoCrPt and CoCrPt-M, where M
= B, Mo, Nb, Ta, W, Cu or alloys thereof. It is
preferable that the grain diameter of the
ferromagnetic crystal grain is in a range of
approximately 2 to 30 nm. Further, the non-magnetic
base material may be made of a ceramic material such
as SiO2, Al2O3 and MgO or an oxide material such as
NiO. On the other hand, the ferromagnetic layer 407
may be made of a CoCr-based magnetic material if not
employing the granular layer structure.

The granular layer structure changes form
depending on fundamental physical constants or
properties, such as cohesive energy, surface energy
and elastic strain energy of the ferromagnetic
crystal grains and the non-magnetic base material.
Accordingly, an extremely large number of
combinations of the magnetic material used for the
ferromagnetic crystal grains and the ceramic or
oxide material used for the non-magnetic base
material exist, and the combination may be
appropriately adjusted to suit the needs.

It is preferable that the granular layer
structure is used with priority for the magnetic
layer 409, in which case the ferromagnetic layer 407
may be made of a CoCr-based magnetic material as
described above. The reason for the preferable use
of the granular layer structure for the magnetic
layer 409 is because, due to the exchange coupling
caused by the provision of the non-magnetic coupling
layer 408, it is the uppermost magnetic layer 409
which contributes most to the noise reduction.

Of course, the ferromagnetic layer 407 and
the magnetic layer 409 are not limited to a single-layer
structure, and a multi-layer structure may be
used for each of the ferromagnetic layer 407 and the
magnetic layer 409.

The non-magnetic coupling layer 408 is
made of Ru, Rh, Ir, Ru-based alloys, Rh-based alloys,
Ir-based alloys, or the like. For example, the non-magnetic
coupling layer 408 may be added with a
ceramic material such as SiO2 and Al2O3 or an oxide
material such as NiO which are used for the granular
layer proposed in the Japanese Laid-Open Patent
Application No.10-149526. The addition of the
ceramic or oxide material to the non-magnetic
coupling layer 408 promotes the epitaxial growth of
the non-magnetic coupling layer 408 and the magnetic
layer 409, thereby further improving the signal-to-noise
(S/N) ratio of the magnetic recording medium.

The protection layer 410 and the lubricant
layer 411 are similar to those of the first and
second embodiments described above.

The ferromagnetic layer 407 may have a
thickness in a range of approximately 2 to 10 nm,
and the magnetic layer 409 may have a thickness in a
range of approximately 5 to 30 nm.

In addition, the magnetization directions
of the ferromagnetic layer 407 and the magnetic
layer 409 may be mutually antiparallel or mutually
parallel.

When making the magnetization directions
of the ferromagnetic layer 407 and the magnetic
layer 409 mutually antiparallel, the non-magnetic
coupling layer 408 desirably is made of a material
selected from a group of Ru, Rh, Ir, Ru-based alloys,
Rh-based alloys and Ir-based alloys, and has a
thickness in a range of approximately 0.4 to 1.0 nm.

When making the magnetization directions
of the ferromagnetic layer 407 and the magnetic
layer 409 mutually parallel, the non-magnetic
coupling layer 408 desirably is made of a material
selected from a group of Ru, Rh, Ir, Ru-based alloys,
Rh-based alloys and Ir-based alloys, and has a
thickness in a range of approximately 0.2 to 0.4 nm
and 1.0 to 1.7 nm. Ru is desirably used for the
non-magnetic coupling layer 408.

The number of exchange layer structures
having the granular layer structure described above
is of course not limited to one, and first and
second exchange layer structures of the second
embodiment described above may be provided with the
granular layer structure. In this case, it is
preferable that the magnetic anisotropy of the
granular layer in the second exchange layer
structure is set smaller than that of the granular
layer in the first exchange layer structure which is
disposed under the second exchange layer structure.
Furthermore, it is preferable that the remanence
magnetization and thickness product of the granular
layer in the second exchange layer structure is set
smaller than that of the granular layer in the first
exchange layer structure which is disposed under the
second exchange layer structure.

Further, the present invention is not
limited to these embodiments, but various variations
and modifications may be made without departing from
the scope of the present invention.

Claims (34)

A magnetic recording medium
characterized by:

at least one exchange layer structure; and

a magnetic layer formed on said exchange layer
structure,

said exchange layer structure comprising:

a ferromagnetic layer; and

a non-magnetic coupling layer provided on
said ferromagnetic layer and under said magnetic
layer.

The magnetic recording medium as
claimed in claim 1, characterized in that said
ferromagnetic layer and said magnetic layer have
antiparallel magnetizations.

The magnetic recording medium as
claimed in claim 1 or 2, characterized in that said
ferromagnetic layer is made of a material selected
from a group consisting of Co, Ni, Fe, Ni-based
alloys, Fe-based alloys, and Co-based alloys
including CoCrTa, CoCrPt and CoCrPt-M, where M = B,
Mo, Nb, Ta, W, Cu or alloys thereof.

The magnetic recording medium as
claimed in any of claims 1 to 3, characterized in
that said non-magnetic coupling layer is made of a
material selected from a group of Ru, Rh, Ir, Ru-based
alloys, Rh-based alloys, and Ir-based alloys.

The magnetic recording medium as
claimed in any of claims 1 to 4, characterized in
that said non-magnetic coupling layer has a
thickness in a range of 0.4 to 1.0 nm.

The magnetic recording medium as
claimed in any of claims 1 to 5, characterized in
that said magnetic layer is made of a material
selected from a group of Co, and Co-based alloys
including CoCrTa, CoCrPt and CoCrPt-M, where M = B,
Mo, Nb, Ta, W, Cu or alloys thereof.

The magnetic recording medium as
claimed in any of claims 1 to 6, further
characterized by:

a substrate; and

an underlayer provided above said substrate,

said exchange layer structure being provided
above said underlayer.

The magnetic recording medium as
claimed in claim 7, further characterized by:

a non-magnetic intermediate layer interposed
between said underlayer and said exchange layer
structure,

said non-magnetic intermediate layer having a
hcp structure alloy selected from a group of CoCr-M,
where M = B, Mo, Nb, Ta, W or alloys thereof, and
having a thickness in a range of 1 to 5 nm.

The magnetic recording medium as
claimed in any of claims 1 to 8, characterized by at
least a first exchange layer structure and a second
exchange layer structure interposed between said
first exchange layer structure and said magnetic
layer, wherein a ferromagnetic layer of said second
exchange layer structure has a magnetic anisotropy
lower than that of a ferromagnetic layer of said
first exchange layer structure, and magnetizations
of the ferromagnetic layers of said first and second
exchange layer structures are antiparallel.

The magnetic recording medium as
claimed in any of claims 1 to 8, characterized by at
least a first exchange layer structure and a second
exchange layer structure interposed between said
first exchange layer structure and said magnetic
layer, wherein a remanent magnetization and
thickness product of a ferromagnetic layer of said
second exchange layer structure is smaller than that
of a ferromagnetic layer of said first exchange
layer structure, and magnetizations of the
ferromagnetic layers of said first and second
exchange layer structures are antiparallel.

a second exchange layer structure provided
between said first exchange layer structure and said
magnetic layer,
wherein a ferromagnetic layer of said second
exchange layer structure has a magnetic anisotropy
lower than that of a ferromagnetic layer of said
first exchange layer structure, and magnetizations
of the ferromagnetic layers of said first and second
exchange layer structures are antiparallel.

A magnetic recording medium
comprising a substrate, and an underlayer disposed
above said substrate, characterized by:

a magnetic layer structure including at least a
bottom ferromagnetic layer provided on the
underlayer and having a remanent magnetization and
thickness product Mriδi, and a top ferromagnetic
layer disposed above the bottom ferromagnetic layer
and having a remanent magnetization and thickness
product Mrjδj, wherein a relationship Mrδ≒Σ(Mriδi-
Mrjδj) is satisfied, where Mrδ denotes a total
remanent magnetization and thickness product of the
magnetic layer structure, so that magnetization
directions of adjacent ferromagnetic layers in the
magnetic layer structure are closely antiparallel.

The magnetic recording medium as
claimed in claim 12, further characterized by:

a non-magnetic coupling layer interposed
between adjacent ferromagnetic layers of the
magnetic layer structure, so that antiparallel
magnetic interaction is induced thereby.

The magnetic recording medium as
claimed in claim 13, characterized in that said non-magnetic
coupling layer is made essentially of Ru
with a thickness of approximately 0.4 to 1.0 nm.

The magnetic recording medium as
claimed in claim 13, characterized in that said non-magnetic
coupling layer is made of a material
selected from a group of Ru, Rh, Ir, Cu, Cr and
alloys thereof.

The magnetic recording medium as
claimed in any of claims 12 to 15, characterized in
that each of the ferromagnetic layers of the
magnetic layer structure is made of a material
selected from a group of Co, Fe, Ni, CoCrTa, CoCrPt
and CoCrPt-M, where M=B, Cu, Mo, Nb, Ta, W and
alloys thereof.

The magnetic recording medium as
claimed in any of claims 12 to 16, characterized in
that at least one of the ferromagnetic layers of the
magnetic layer structure is made up of a plurality
of ferromagnetic layers which are in contact with
each other and ferromagnetically coupled.

The magnetic recording medium as
claimed in any of claims 12 to 17, characterized in
that the Mrjδj of the top ferromagnetic layer is
largest among remanent magnetization and thickness
products of other ferromagnetic layers of the
magnetic layer structure.

A method of magnetically recording
information on a magnetic recording medium,
characterized by:

a step of switching magnetization direction of
at least one of ferromagnetic layers which form a
magnetic layer structure of the magnetic recording
medium and have antiparallel magnetization
directions.

A method of producing a magnetic
recording medium having a substrate, an underlayer
and a magnetic layer structure, characterized by the
steps of:

(a) forming the magnetic layer structure to
include at least a bottom ferromagnetic layer
provided on the underlayer and having a remanent
magnetization and thickness product Mriδi, and a top
ferromagnetic layer disposed above the bottom
ferromagnetic layer and having a remanent
magnetization and thickness product Mrjδj, wherein a
relationship Mrδ≒Σ(Mriδi-Mrjδj) is satisfied,
where Mrδ denotes a total remanent magnetization
and thickness product of the magnetic layer
structure, so that magnetization directions of
adjacent ferromagnetic layers in the magnetic layer
structure are closely antiparallel; and

(b) forming the underlayer and the magnetic
structure by sequential sputtering.

A magnetic recording medium
characterized by:

at least one exchange layer structure and a
magnetic layer provided on the exchange layer
structure, said exchange layer structure including a
ferromagnetic layer and a non-magnetic coupling
layer provided on the ferromagnetic layer; and

a magnetic bonding layer provided between the
ferromagnetic layer and the non-magnetic coupling
layer and/or between the non-magnetic coupling layer
and the magnetic layer,

said magnetic bonding layer having a
magnetization direction parallel to the
ferromagnetic layer and the magnetic layer.

The magnetic recording medium as
claimed in claim 21, characterized in that said
magnetic bonding layer is made of a material
different from those of the ferromagnetic layer and
the magnetic layer.

The magnetic recording medium as
claimed in claim 21 or 22, characterized in that an
upper magnetic bonding layer and a lower magnetic
bonding layer are respectively provided above and
below the non-magnetic coupling layer, and an
exchange coupling between the upper magnetic bonding
layer and the lower magnetic bonding layer is larger
than an exchange coupling between the magnetic layer
and the ferromagnetic layer.

The magnetic recording medium as
claimed in any of claims 21 to 23, characterized in
that said magnetic bonding layer is made of a
material selected from a group of Co, Fe, Fe-based
alloys, Ni-based alloys, and Co-based alloys
including CoCrTa, CoCrPt and CoCrPt-M, where M = B,
Mo, Nb, Ta, W, Cu or alloys thereof.

The magnetic recording medium as
claimed in claim 24, characterized in that a Co or
Fe concentration of the magnetic bonding layer is
higher than a Co or Fe concentrations of the
ferromagnetic layer and the magnetic layer.

A magnetic recording medium
characterized by:

at least one exchange layer structure; and

a magnetic layer formed on said exchange layer
structure,

said exchange layer structure comprising a
ferromagnetic layer, and a non-magnetic coupling
layer provided on said ferromagnetic layer and under
said magnetic layer,

said ferromagnetic layer and said magnetic
layer having antiparallel magnetizations,

said non-magnetic coupling layer being made of
a Ru-M3 alloy, where M3 is an added element or alloy,
and a lattice mismatch between said non-magnetic
coupling layer and said magnetic layer and said
ferromagnetic layer respectively disposed above and
below said non-magnetic coupling layer is adjusted
to approximately 6% or less by addition of M3.

A magnetic recording medium
characterized by:

at least one exchange layer structure; and

a magnetic layer formed on said exchange layer
structure,

said exchange layer structure comprising a
ferromagnetic layer, and a non-magnetic coupling
layer provided on said ferromagnetic layer and under
said magnetic layer,

said ferromagnetic layer and said magnetic
layer having antiparallel magnetizations,

said non-magnetic coupling layer being made of
a Ru-M3 alloy, where M3 = Co, Cr, Fe, Ni, Mn or
alloys thereof.

The magnetic recording medium as
claimed in claim 26 or 27, characterized in that an
amount of the element M3 added to Ru is 50 at% or
less for Co, 50 at% or less for Cr, 60 at% or less
for Fe, 10 at% or less for Ni, and 50 at% or less
for Mn.

A magnetic recording medium
comprising: a substrate; an underlayer disposed
above the substrate; and a magnetic recording layer
disposed above the underlayer, characterized in
that:

said magnetic recording layer has a multi-layer
structure which is separated into at least upper and
lower layers by a non-magnetic separation layer,

said non-magnetic separation layer is made of a
material selected from a group of Ru, Rh, Ir and
alloys thereof,

the upper and lower layers of the multi-layer
structure separated by the non-magnetic separation
layer have magnetization directions which are
mutually parallel.

The magnetic recording medium as
claimed in claim 29, characterized in that said non-magnetic
separation layer has a thickness in a range
of approximately 0.2 to 0.4 nm or approximately 1.0
to 1.7 nm.

The magnetic recording medium as
claimed in claim 29 or 30, characterized in that
said magnetic recording layer is separated into an
upper first magnetic recording layer and a lower
second magnetic recording layer by said non-magnetic
separation layer, and a thickness ratio of a
thickness of the first magnetic recording layer to a
thickness of the second magnetic recording layer is
in a range of approximately 5:5 to 7:3 when the
first and second magnetic recording layers are made
of the same magnetic material.

A magnetic recording medium
characterized by:

at least one exchange layer structure; and

a magnetic layer provided on the exchange layer
structure,

said exchange layer structure including a
ferromagnetic layer and a non-magnetic coupling
layer provided on the ferromagnetic layer,

at least one of said ferromagnetic layer and
said magnetic layer having a granular layer
structure in which ferromagnetic crystal grains are
uniformly distributed within a non-magnetic base
material.

The magnetic recording medium as
claimed in claim 32, characterized in that said
ferromagnetic crystal grains are made of a material
selected from a group of Co, Ni, Fe, Ni-based alloys,
Fe-based alloys, and Co-based alloys including
CoCrTa, CoCrPt and CoCrPt-M, where M = B, Mo, Nb, Ta,
W, Cu and alloys thereof.

The magnetic recording medium as
claimed in claim 32 or 33, characterized in that
said non-magnetic base material is made of a
material selected from a group of ceramic materials
and oxide materials.